Allow removing column comment on some platforms

Some platforms currently do not remove a comment on a column on table alteration.
This PR fixes this. Also updated platform and functional schema tests for this.
Besides that <code>OraclePlatform</code> now does not generate a useless <code>ALTER TABLE MODIFY</code> SQL snippet when only the column comment has changed, as this is handled seperately through <code>COMMENT ON COLUMN</code> SQL statement.