Remove gitlab-shell from test setup

What does this MR do?

Remove GitLab Shell from the test setup, as the required parts are absorbed by Gitaly.

Screenshots

$ bundle exec rspec spec/lib/gitlab/git
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

==> Setting up Gitaly...
Checking gitaly-ruby bundle...
Warning: the running version of Bundler (1.17.2) is older than the version that created the lockfile (1.17.3). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.
The Gemfile's dependencies are satisfied
Trying to connect to gitaly: .... OK
    Gitaly set up in 0.704779359 seconds...

==> Setting up GitLab Elasticsearch Indexer...
    GitLab Elasticsearch Indexer set up in 0.000106791 seconds...

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team