Bug 1735260 - Remove unused PAINT_BUILD_LAYERS_TIME metric. r=gfx-reviewers,sotaro

Differential Revision: https://phabricator.services.mozilla.com/D128153

toolkit/components/telemetry/Histograms.json

@@ -4835,17 +4835,6 @@
     "high": 1000,
     "n_buckets": 50
   },
-  "PAINT_BUILD_LAYERS_TIME" : {
-    "record_in_processes": ["content"],
-    "products": ["firefox", "fennec"],
-    "alert_emails": ["gfx-telemetry-alerts@mozilla.com", "mwoodrow@mozilla.com"],
-    "bug_numbers": [1473908],
-    "expires_in_version": "never",
-    "description": "Time spent in building layers in milliseconds",
-    "kind": "exponential",
-    "high": 1000,
-    "n_buckets": 50
-  },
   "PAINT_RASTERIZE_TIME" : {
     "record_in_processes": ["main", "content"],
     "products": ["firefox", "fennec"],