www/py-flask-migrate: Revert c6f6acd8e96e1ad7ae7c12e9943bb102995e94a2

This port is waiting for py-alembic with py-sqlalchemy14.

Reference: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=258790#c4

		https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=257371#c6