Clean up gitlab-shell install-from-source instructions

What does this MR do?

GitLab Shell is an external component of gitlab that has gone through a number of changes recently. The install-from-source docs and code have not been updated to fully reflect that, so this MR makes a number of changes to bring it up to date:

• GitLab Shell no longer talks directly to Redis, so doesn't need REDIS_URL
• The SKIP_STORAGE_VALIDATION envvar no longer has any effect, so we don't need to set it
• GitLab Shell is no longer written in Ruby, so we don't need to include so many caveats in the documentation

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Noticed while reviewing gitlab-shell!388 (merged)