Remove UserAssigned MSI (#12900)

Author: ahermas

specification/datashare/resource-manager/Microsoft.DataShare/stable/2020-09-01/DataShare.json

@@ -4698,8 +4698,7 @@
         "type": {
           "description": "Identity Type",
           "enum": [
-            "SystemAssigned",
-            "UserAssigned"
+            "SystemAssigned"
           ],
           "type": "string",
           "x-ms-enum": {