Drop Bitbucket support

since only the Bitbucket Server product supports signed payloads,
bitbucket.org does not.  And we are requiring signed payloads.

awx/main/models/credential/__init__.py

@@ -1001,22 +1001,6 @@ ManagedCredentialType(
     },
 )
 
-ManagedCredentialType(
-    namespace='bitbucket_token',
-    kind='token',
-    name=ugettext_noop('Bitbucket Personal Access Token'),
-    managed_by_tower=True,
-    inputs={
-        'fields': [{
-            'id': 'token',
-            'label': ugettext_noop('Token'),
-            'type': 'string',
-            'secret': True,
-        }],
-        'required': ['token'],
-    },
-)
-
 ManagedCredentialType(
     namespace='insights',
     kind='insights',