OpenAPI: Add RemoveSchemas REST update type

[gaborkaszab]

No description provided.

[gaborkaszab]

Similarly to removing unused specs, I started working on removing unused schemas. This PR is similar to #10846
As an author of the other PR, @amogh-jahagirdar would you mind taking a look?

[amogh-jahagirdar]

@gaborkaszab I agree with this spec change, I think it's very important to maintaining table metadata sizes for cases where there's frequent schema evolution but we'll need to put this to a discuss/vote thread.

Could you send an email on the dev list? https://lists.apache.org/thread/2lo59wkhn8cm29wmokfmg2f18934qnln as an example

[gaborkaszab]

but we'll need to put this to a discuss/vote thread.

Thanks for taking a look, @amogh-jahagirdar !
Yes, I was aware of the need of at least a [VOTE] thread, but wasn't sure about the order of reviewing the PR vs opening a mail thread. I started a [DISCUSS] thread now, and if that goes well I guess the [VOTE] is just formality.