fix(groups): Run discussion reply migration regardless if groups plugin is enabled.

This upgrade must be run even if the groups plugin is disabled because the
script will be removed in Elgg 1.10 and we don't want anyone to get stuck
with old annotation replies just because the groups plugin was not enabled
when site was upgraded from 1.8.

Fixes #6729
Merge pull request #6823 from juho-jaakkola/force_discussion_upgrade

Run discussion reply migration regardless if groups plugin is enabled