Revert "Make WebKit Win10 part of win-ci."

This reverts commit e5d0061d.

Reason for revert: only switch after confirming WAI

Original change's description:
> Make WebKit Win10 part of win-ci.
> 
> BUG=790343
> 
> Change-Id: I64d509837aaf84043e1e3e749bcda748e0c42da4
> Reviewed-on: https://chromium-review.googlesource.com/c/1260012
> Reviewed-by: Ryan Tseng <hinoka@chromium.org>
> Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#596409}

TBR=hinoka@chromium.org,jchinlee@chromium.org

Change-Id: I0f3c5228ab4fe404c31244bbcf92f9328ac3b9f1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 790343
Reviewed-on: https://chromium-review.googlesource.com/c/1263051Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596884}

infra/config/global/cr-buildbucket.cfg

@@ -2396,7 +2396,7 @@ buckets {
     builders {
       name: "WebKit Win10"
       dimensions: "os:Windows-10"
-      mixins: "win-ci"
+      mixins: "webkit-ci"
     }
     builders {
       name: "win32-dbg"