Removing redundant constant. (#4588)

dhermes · web-flow · commit 925907bd123f · 2017-12-13T14:59:39.000-08:00
It looks like it was moved to `google.api_core.gapic_v1.client_info`
and during the move the delete was forgotten.
diff --git a/packages/google-api-core/google/api_core/gapic_v1/method.py b/packages/google-api-core/google/api_core/gapic_v1/method.py
@@ -23,7 +23,6 @@
 from google.api_core import timeout
 from google.api_core.gapic_v1 import client_info
 
-METRICS_METADATA_KEY = 'x-goog-api-client'
 USE_DEFAULT_METADATA = object()
 DEFAULT = object()
 """Sentinel value indicating that a retry or timeout argument was unspecified,
