ruby: Remove version file

We have recently rewritten the way our Protobuf Gem is generated. As part of that rewrite we have removed all generated Protobuf code from our codebase as it is not needed anymore and instead generated ad-hoc when building the Gem.

Part of this generated code is also version.rb, which contains the current Gitaly version. This file is being written by the Release tools bot when tagging a new release, but it is not really required as the same version is also present in our VERSION file. We thus also started to generate this file by seeding it with the contents of our VERSION and have subsequently removed it from our tree in 32691316 (ruby: Remove stale generated Protobuf code, 2022-10-27).

Given that the Release tools bot hasn't yet been upgraded though to stop writing this file it was reintroduced in a later commit. The change to the bot has since landed though, which effectively means that the file is now stale and won't ever be updated anymore. So let's remove it.