Correct `op.drop_constraint` parameters

`drop_constraint` method require `type_` parameter, when called with
MySQL.

> type_ – optional, required on MySQL. can be "foreignkey", "primary",
"unique", or "check".

Change-Id: I069a74ea60ba678fb128f27858db169f4dac89a6
Closes-Bug: #1354463
Merge "Correct `op.drop_constraint` parameters"