update custom_injectors and remove unused import

awx/main/models/credential.py

@@ -54,7 +54,6 @@
 )
 from awx.main.models import Team, Organization
 from awx.main.utils import encrypt_field
-from awx_plugins.credentials import injectors as builtin_injectors
 from awx_plugins.interfaces._temporary_private_licensing_api import detect_server_product_name
 
 
@@ -450,7 +449,7 @@ def from_db(cls, db, field_names, values):
             native = ManagedCredentialType.registry[instance.namespace]
             instance.inputs = native.inputs
             instance.injectors = native.injectors
-            instance.custom_injectors = native.custom_injectors
+            instance.custom_injectors = getattr(native, 'custom_injectors', None)
         return instance
 
     def get_absolute_url(self, request=None):