MYSQL: added support for drop check/constraint

MYSQL: added support for drop check/constraint

Added support for DROP CHECK constraint which is required by MySQL 8.0.16
to drop a CHECK constraint; MariaDB supports plain DROP CONSTRAINT.  The
logic distinguishes between the two syntaxes by checking the server version
string for MariaDB presence.    Alembic migrations has already worked
around this issue by implementing its own DROP for MySQL / MariaDB CHECK
constraints, however this change implements it straight in Core so that its
available for general use.   Pull request courtesy Hannes Hansen.

Fixes: #4650
Closes: #4659
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4659
Pull-request-sha: 5b654a55e23c2ca498ca3b1cae4f53859e93e8f7

Change-Id: I967710f890722f11cf1f40406adbb17464d16194