Revert "Add validate to metrics backends"

This reverts commit 6d237a5.

Author: dcramer

src/sentry/metrics/base.py

@@ -26,6 +26,3 @@ def incr(self, key, instance=None, tags=None, amount=1, sample_rate=1):
 
     def timing(self, key, value, instance=None, tags=None, sample_rate=1):
         raise NotImplementedError
-
-    def validate(self):
-        pass

src/sentry/utils/runner.py

@@ -353,7 +353,6 @@ def validate_backends():
         app.buffer,
         app.digests,
         app.nodestore,
-        app.metrics,
         app.quotas,
         app.ratelimiter,
         app.search,