Remove old deprecation classes for 3.14 release (#8664)

When DRF 3.14 is released, these exception classes will be meaningless,
so we can delete them (this has always been done).

A previous PR removed the last incidence of `RemovedInDRF313Warning`,
but didn't outright delete the class for fear of shipping a breaking
change: #8589

Author: DavidCain

rest_framework/__init__.py

@@ -29,13 +29,5 @@
     default_app_config = 'rest_framework.apps.RestFrameworkConfig'
 
 
-class RemovedInDRF313Warning(DeprecationWarning):
-    pass
-
-
-class RemovedInDRF314Warning(PendingDeprecationWarning):
-    pass
-
-
 class RemovedInDRF315Warning(PendingDeprecationWarning):
     pass