Fix incorrect chromium.webrtc dimensions for win7 and win8.

These are called WindowsServer2008 and Windows-2012ServerR2-SP0 and
stuff like that nowadays. This CL just drops the OS dimension since
bots.cfg ensures each builder runs on the right OS, anyway.

Tbr: jbudorick@chromium.org
Bug: 877018
Change-Id: Ib71ee5f1d26ed90b38fb0b9868994f4bd0c29ad3
Reviewed-on: https://chromium-review.googlesource.com/c/1354924Reviewed-by: Patrik Höglund <phoglund@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612183}

infra/config/global/cr-buildbucket.cfg

@@ -3516,31 +3516,11 @@ buckets {
 
     # Win
 
-    builders {
-      mixins: "win"
-      name: "WebRTC Chromium FYI Win Builder"
-      dimensions: "os:Windows-10"
-    }
-    builders {
-      mixins: "win"
-      name: "WebRTC Chromium FYI Win Builder (dbg)"
-      dimensions: "os:Windows-10"
-    }
-    builders {
-      mixins: "win"
-      name: "WebRTC Chromium FYI Win10 Tester"
-      dimensions: "os:Windows-10"
-    }
-    builders {
-      mixins: "win"
-      name: "WebRTC Chromium FYI Win7 Tester"
-      dimensions: "os:Windows-7"
-    }
-    builders {
-      mixins: "win"
-      name: "WebRTC Chromium FYI Win8 Tester"
-      dimensions: "os:Windows-8.1"
-    }
+    builders { mixins: "win" name: "WebRTC Chromium FYI Win Builder" }
+    builders { mixins: "win" name: "WebRTC Chromium FYI Win Builder (dbg)" }
+    builders { mixins: "win" name: "WebRTC Chromium FYI Win10 Tester" }
+    builders { mixins: "win" name: "WebRTC Chromium FYI Win7 Tester" }
+    builders { mixins: "win" name: "WebRTC Chromium FYI Win8 Tester" }
   }
 }