ruby: Remove UserCherryPick implementation
In [1] the feature flag for the Go implementation is removed, making the Go implementation the only possible code path for handling the UserCherryPick RPC.
This was done in version 13.12. In this change, included in 14.0, the Ruby implementation for this RPC is removed, since it's no longer can be called.
- 4ad92cb4 (operations: Drop GoUserCherryPick feature flag, 2021-04-26)
Fixes: #3281 (closed)