Make _check_unavailable always pass for INTERNAL errors

Author: odeke-em

google/cloud/spanner_v1/_helpers.py

@@ -588,14 +588,9 @@ def _check_rst_stream_error(exc):
 
 
 def _check_unavailable(exc):
-    resumable_error = (
-        any(
-            resumable_message in exc.message
-            for resumable_message in ("Service unavailable",)
-        ),
-    )
-    if not resumable_error:
-        raise
+    # For UNAVAILABLE, we don't need to check against the
+    # messages as these should just be noop and retryable always.
+    return
 
 
 def _metadata_with_leader_aware_routing(value, **kw):