lib: fix table names

lib: fix table names

Migration 0042 (introduced in 0686a73) renamed the "Comment" model
to "PatchComment", and some of the table names changed in a51e7cf
were already correct.

This reverts a51e7cf and removes a nonexistent table name to resolve
the following error:

  $ psql -f lib/sql/grant-all.postgres.sql DATABASE_NAME
  BEGIN
  psql:lib/sql/grant-all.postgres.sql:38: ERROR:  relation
    "patchwork_comment" does not exist

Fixes: a51e7cfd95e2 ("lib: Correct PatchComment table")
Fixes: 7b967db4e12f ("lib: Update DB permissions to reflect model changes")
Cc: stephen@that.guru
Closes: #396
Signed-off-by: Ali Alnubani <alialnu@nvidia.com>
(cherry picked from commit 5b53f46def5fc4961bfa77a8abb3e968ae839c59)