Mostly drop old comment schemas

Mostly drop old comment schemas

This removes most of the pre-CommentStore text columns, and the
$wgCommentTableSchemaMigrationStage setting that used to determine
whether the columns were used.

rev_comment remains in the code, as on Wikimedia wikis the revision
table is too large to alter at this time. A future change will combine
that with the removal of rev_user_text, rev_content_model, and
rev_content_format (and the addition of rev_comment_id and rev_actor).

CommentStore's constructor continues to take a $stage parameter, and
continues to have the logic for handling it, for the benefit of
extensions that might need their own migration process.

Bug: T166733
Change-Id: I1479c73774e01ead1490adf6128f820c09bce9d4