Syntax:
DROP [ONLINE|OFFLINE] INDEX index_name ON tbl_name

DROP INDEX drops the index named index_name from the table tbl_name.
This statement is mapped to an ALTER TABLE statement to drop the index.
See [HELP ALTER TABLE].

To drop a primary key, the index name is always PRIMARY, which must be
specified as a quoted identifier because PRIMARY is a reserved word:

DROP INDEX `PRIMARY` ON t;

URL: http://dev.mysql.com/doc/refman/5.5/en/drop-index.html

