Remove rpm-plugin-ima in Dockerfile

This plugin of ours, when installed in a container, currently causes
transaction errors (simply because IMA isn't supported in containers
currently).

This isn't immediately visible unless we'd run a second dnf transaction
in the Dockerfile but it's simply unnecessary, we don't need the plugin
for anything.

The reason it's installed is through the ima-evm-utils-devel package
(which pulls in ima-evm-utils-libs which pulls in the plugin), see
RHBZ#2292869 for details.

tests/Dockerfile.fedora

@@ -71,7 +71,8 @@ RUN which dnf5 || \
     rpm -e --nodeps --nodb \
 	rpm \
 	rpm-libs \
-	rpm-build-libs
+	rpm-build-libs \
+	rpm-plugin-ima
 
 WORKDIR /
 CMD /bin/bash