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
Fix binary size handling migration to Active Record

review1017002
(transplanted from 863dfb6f9a25e00ef711dee6de50e60b23664d59)

--HG--
extra : transplant_source : %86%3D%FBo%9A%25%E0%0E%F7%11%DE%E6%DEP%E6%0B%23fMY