[RUN AS-IF-FOSS] Use upstream ExpectOffense module

What does this MR do?

The one we have is a copy from rubocop-rspec which only changes the default filename which we don't need

Removes manual includes of ExpectOffense modules

Remove custom ExpectOffense module

Related issue: #222306 (closed)

Sanity check

Before:

$ ss spec/rubocop/
Running via Spring preloader in process 83259
Run options: include {:focus=>true}

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

==> Setting up Gitaly...
Checking gitaly-ruby Gemfile...
Checking gitaly-ruby bundle...
The Gemfile's dependencies are satisfied
Trying to connect to gitaly: ........ OK
Trying to connect to praefect: .. OK
    Gitaly set up in 1.493702 seconds...

==> Setting up GitLab Workhorse...
    GitLab Workhorse set up in 0.000266 seconds...

==> Setting up GitLab Elasticsearch Indexer...
    GitLab Elasticsearch Indexer set up in 0.00018 seconds...
.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished in 13.63 seconds (files took 2.22 seconds to load)
593 examples, 0 failures

After:

$ ss spec/rubocop/
Running via Spring preloader in process 83127
Run options: include {:focus=>true}

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

==> Setting up Gitaly...
Checking gitaly-ruby Gemfile...
Checking gitaly-ruby bundle...
The Gemfile's dependencies are satisfied
Trying to connect to gitaly: ....... OK
Trying to connect to praefect: .. OK
    Gitaly set up in 1.333614 seconds...

==> Setting up GitLab Workhorse...
    GitLab Workhorse set up in 0.0003 seconds...

==> Setting up GitLab Elasticsearch Indexer...
    GitLab Elasticsearch Indexer set up in 0.000182 seconds...
.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished in 14.1 seconds (files took 2.11 seconds to load)
593 examples, 0 failures