schema: drop entity object types

SC_ENTITY_FOO is used instead of SC_FOO when a privilege is granted to
an entire object class, not an individual object (object id is set to ''
in the _priv system space). Introduction of this new concept made the
access checking code rather confusing, especially the part converting
entity types to object types and back, and complicated addition of new
schema object types.

Actually, there's no point in maintaining separate schema object types
for entities. Instead, we can simply add a flag to the priv_def struct
saying that the object id stored in the struct is meaningless and the
privilege should be applied to an entire object class. This simplifies
the code quite a bit and makes introduction of new schema object types
must easier.

Needed for #8803

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring