Partially remove quota-class logic from nova.quotas and test_quotas

Continuation of quota-class removal started in
I1110022d6f628d03aaf363da707f2d2ef1600437.

* No need to have quota-class in FakeContext since quota-class isn't in
  the real context anymore.
* Don't check for quota_class _DEFAULT_QUOTA_NAME='default' in
  get_defaults
* Remove get_by_class
* Remove get_class_quotas
* Full removal is in the subsequent patch.

Change-Id: Ia563ac3341b980c1b5a4a45fe73770ddc27acc91
Merge "Partially remove quota-class logic from nova.quotas and test_quotas"