Update Win8 CQ config to 100% GN-only.

This patch follows up on #333386 as it looks
like things are working correctly now.
With this patch, the "win8_chromium_ng" bot
will be completely on (100%, blocking), and
the "win8_chromium_gn_rel" and "win8_chromium_rel"
bots will be completely off.

TBR=sergeyberezin@chromium.org, phajdan.jr@chromium.org
BUG=497423
NOTRY=true

Review URL: https://codereview.chromium.org/1169013002

Cr-Commit-Position: refs/heads/master@{#333414}

infra/config/cq.cfg

@@ -84,22 +84,11 @@ verifiers {
     }
     buckets {
       name: "tryserver.chromium.win"
-      builders {
-        name: "win8_chromium_gn_rel"
-        experiment_percentage: 0.1
-      }
-      builders {
-        name: "win8_chromium_ng"
-        experiment_percentage: 0.5
-      }
       builders {
         name: "win_chromium_compile_dbg_ng_exp"
         experiment_percentage: 0.1
       }
-      builders {
-        name: "win8_chromium_rel"
-        experiment_percentage: 0.1
-      }
+      builders { name: "win8_chromium_ng" }
       builders { name: "win_chromium_compile_dbg_ng" }
       builders { name: "win_chromium_rel_ng" }
       builders { name: "win_chromium_x64_rel_ng" }