Support instant DROP CHECK CONSTRAINT

When MDEV-7563 introduced support for the CHECK constraint, it failed to add the flag for DROP CONSTRAINT to INNOBASE_INPLACE_IGNORE. Thus, dropping a CHECK constraint will unnecessarily require the table to be rebuilt, with ALGORITHM=COPY.

ADD [CONSTRAINT] CHECK does not strictly require a rebuild. As part of MDEV-16291 we can implement validation of all rows without copying the table.