[Bug 1592129] remove subclass loading and driver delegation from Schema->new.

[Bug 1592129] remove subclass loading and driver delegation from Schema->new.

Bugzilla::DB::Schema->new() was both a normal constructor and also a
class-loading factory method. It is simpler to just do the class loading at the
call-site (in Bugzilla::DB::_bz_schema).

It's not very likely extensions relied on this behavior so this should be a good
change.