[schema2sql] Drop most of the DB DROP related code
This code is dead for a while, it has been superseded by
`sql_drop_all_user_tables`.
Related to #14050899
usr/lib/python2*/*-packages/cubicweb/dataimport/
usr/lib/python2*/*-packages/cubicweb/server/
usr/lib/python2*/*-packages/cubicweb/hooks/
usr/lib/python2*/*-packages/cubicweb/sobjects/
usr/lib/python2*/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/