Remove Ruby implementations of gitlab-shell commands

Once all the functionality in &706 (closed) is added and validated, we'll need to remove lots of ruby code from gitlab-shell. Hopefully all of it :D

Also, as mentioned by @reprazent here (!318 (comment 197620284)):

I think at some point after the migration, we would need to have this buildCommand call return errors: In case something calls gitlab-shell in a way we don't expect.

So instead of checking for the cmd to be nil we should check for an error.

Right now, we're counting on the ruby fallback to deal with that, which is okay for now, but could you create an issue to remove the ruby-fallbacks and mention that issue here?

Since we're removing the ruby implementations in this issue, also change the buildCommand to return an error.

Distinct steps:

• Remove feature flag and fallback code: !336 (merged)
• Decouple build- and install- time code from files in lib/
  • Shell: decouple bin/compile and bin/install from lib/: !340 (merged)
  • Follow-up issue: #213 (closed)
  • Shell: decouple bin/check from lib/: !341 (merged)
  • Shell: remove bin/create-hooks: !342 (merged)
• Prepare for bin/compile to be removed
  • Omnibus: use make build: omnibus-gitlab!3653 (merged)
  • Charts: use make build: gitlab-org/build/CNG!277 (merged)
  • Install from source: use make build: gitlab!18059 (merged)
  • GDK: Use make build: gitlab-development-kit!837 (merged)
• Remove runtime Ruby code: !346 (merged)
• Remove install & test-time ruby code: #213 (closed)
• Further updates to omnibus?
• Further updates to charts?
• Further updates to GDK?
• Further updates to gitlab: gitlab!18820 (merged)
• Move go/* up one level: !350 (merged)