Fix MySQL drop index query

"ON" is required for MySQL
(transplanted from 0204193c8ed628dbc48dec8d4ec001f66a00f1f8)

--HG--
extra : transplant_source : %02%04%19%3C%8E%D6%28%DB%C4%8D%EC%8DN%C0%01%F6j%00%F1%F8