Add missing relation types items in schema global variables Some of these global variables are used, in particular, to control schema views (web/views/schema.py). While they are currently not properly documented (see #3360868), their definition are currently incomplete (e.g. one could reasonably expect to find ``cw_schema`` in ``SYSTEM_RTYPES``). Closes #3486114.

sql('DROP TABLE "deleted_entities"')
sql('ALTER TABLE "entities" DROP COLUMN "mtime"')
sql('ALTER TABLE "entities" DROP COLUMN "source"')