chore: remove obsolete release job configs

[alicejli]

We have moved the release jobs to Google3. The requirements.txt
is now unused here.

Internal bug: b/296905003

Note that I had to add entries in owlbot.py because they are templated files.

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .github/CODEOWNERS
• .kokoro/release/bump_snapshot.cfg
• .kokoro/release/common.cfg
• .kokoro/release/common.sh
• .kokoro/release/drop.cfg
• .kokoro/release/drop.sh
• .kokoro/release/promote.cfg
• .kokoro/release/promote.sh
• .kokoro/release/publish_javadoc.cfg
• .kokoro/release/publish_javadoc.sh
• .kokoro/release/publish_javadoc11.cfg
• .kokoro/release/publish_javadoc11.sh
• .kokoro/release/snapshot.cfg
• .kokoro/release/snapshot.sh
• .kokoro/release/stage.cfg
• .kokoro/release/stage.sh

[alicejli]

Kokoro-Test: Samples keeps failing:

[INFO] Running pubsublite.QuickStartIT
com.google.api.gax.rpc.ResourceExhaustedException: io.grpc.StatusRuntimeException: RESOURCE_EXHAUSTED: Failed to allocate quota for pubsublite.googleapis.com/reservation_throughput_allocated.
	at com.google.api.gax.rpc.ApiExceptionFactory.createException(ApiExceptionFactory.java:100)
	at com.google.api.gax.grpc.GrpcApiExceptionFactory.create(GrpcApiExceptionFactory.java:98)
	at com.google.api.gax.grpc.GrpcApiExceptionFactory.create(GrpcApiExceptionFactory.java:66)
	at com.google.api.gax.grpc.GrpcExceptionCallable$ExceptionTransformingFuture.onFailure(GrpcExceptionCallable.java:97)
	at com.google.api.core.ApiFutures$1.onFailure(ApiFutures.java:84)
	at com.google.common.util.concurrent.Futures$CallbackListener.run(Futures.java:1127)
	at com.google.common.util.concurrent.DirectExecutor.execute(DirectExecutor.java:31)
	at com.google.common.util.concurrent.AbstractFuture.executeListener(AbstractFuture.java:1286)
	at com.google.common.util.concurrent.AbstractFuture.complete(AbstractFuture.java:1055)
	at com.google.common.util.concurrent.AbstractFuture.setException(AbstractFuture.java:807)
	at io.grpc.stub.ClientCalls$GrpcFuture.setException(ClientCalls.java:568)
	at io.grpc.stub.ClientCalls$UnaryStreamToFuture.onClose(ClientCalls.java:538)
	at io.grpc.PartialForwardingClientCallListener.onClose(PartialForwardingClientCallListener.java:39)
	at io.grpc.ForwardingClientCallListener.onClose(ForwardingClientCallListener.java:23)
	at io.grpc.ForwardingClientCallListener$SimpleForwardingClientCallListener.onClose(ForwardingClientCallListener.java:40)
	at com.google.api.gax.grpc.ChannelPool$ReleasingClientCall$1.onClose(ChannelPool.java:570)
	at io.grpc.internal.DelayedClientCall$DelayedListener$3.run(DelayedClientCall.java:489)
	at io.grpc.internal.DelayedClientCall$DelayedListener.delayOrExecute(DelayedClientCall.java:453)
	at io.grpc.internal.DelayedClientCall$DelayedListener.onClose(DelayedClientCall.java:486)
	at io.grpc.internal.ClientCallImpl.closeObserver(ClientCallImpl.java:574)
	at io.grpc.internal.ClientCallImpl.access$300(ClientCallImpl.java:72)
	at io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1StreamClosed.runInternal(ClientCallImpl.java:742)
	at io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1StreamClosed.runInContext(ClientCallImpl.java:723)
	at io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37)
	at io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:133)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:750)
Caused by: io.grpc.StatusRuntimeException: RESOURCE_EXHAUSTED: Failed to allocate quota for pubsublite.googleapis.com/reservation_throughput_allocated.
	at io.grpc.Status.asRuntimeException(Status.java:533)
	... 17 more

It's failing for other PRs as well and likely is a quota issue. @davidtorres is the quota managed through a config that we can change easily? Otherwise I'll look to rerun the kokoro test in a couple hours in hopes the quota will be updated then.

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