Commit comments should not be imported

There are lots of redundant automatic comments which were posted on trac whenever a commit mentions a ticket number. These should instead be converted into proper links from the commit to the ticket and the comments removed.

For example:

https://gitlab.staging.haskell.org/ghc/ghc/issues/13545#note_111375

I wrote some code previously to identify which commits these were:

https://github.com/danpalmer/trac-to-phabricator/blob/e9071c8b49a33c521bc891d531c700c6206c1461/src/Trac.hs#L285

As a result, many useless user accounts are created.

https://gitlab.staging.haskell.org/ghc/ghc/project_members?utf8=%E2%9C%93&search=pickering