Optimize fetching issues closed by a merge request

Instead of running ClosingIssueExtractor for every commit in a merge
request we can gather all the commit messages (and the merge request
description), concatenate all this together and then run
ClosingIssueExtractor only once.

The result of this is that MergeRequest#closes_issues is now between
3.5x and 4x faster than the old setup. Using a merge request with 10
commits (each referencing a number of issues to close) this reduced the
call duration from around 200 milliseconds to around 50 milliseconds.

As a result of these changes the Jira related tests for
MergeRequest#closes_issues have been removed. These tests stubbed
Commit#closes_issues meaning that the only code that was really tested
was the call to Array#uniq to filter out duplicate issues. As this code
is no longer used (nor present) the corresponding tests were removed.

Related: gitlab-org/gitlab-ce#12419