chore(gapic): remove _is_tracing_supported dummy class variable

Author: chalmerlowe

packages/google-api-core/google/api_core/gapic_v1/method.py

@@ -125,8 +125,6 @@ class _GapicCallable(object):
             additional metadata will be passed to the RPC method.
     """
 
-    _is_tracing_supported = True
-
     def __init__(
         self,
         target,