Simplify v2 schema lookup

We don't need to look at the container of available schemas in order
to get the one we want. Remove glanceclient.exc.SchemaNotFound as it can
no longer be raised.

Change-Id: Ib49ad58c4fdfc9bc9f535115674d92040a97db65

glanceclient/exc.py

@@ -33,11 +33,6 @@ class EndpointNotFound(Exception):
     pass
 
 
-class SchemaNotFound(KeyError):
-    """Could not find schema"""
-    pass
-
-
 class InvalidEndpoint(ValueError):
     """The provided endpoint could not be used"""
     pass