Removing redundant constant. (#4588)

It looks like it was moved to `google.api_core.gapic_v1.client_info`
and during the move the delete was forgotten.

Author: dhermes

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,