Remove the win10 mixin from ToTWin64

Example failing build:
https://ci.chromium.org/ui/p/chrome/builders/ci/ToTWin64/8096/overview

This buildbot is chrome-branded, so it uses the
chrome-tester-service-account mixin. I believe this means it uses
separate pools of machines. There are no bots in that machine pool that
match the precise Windows 10 version specified in the Windows 10 mixin.
The ToTWin bot is the 32-bit analogue of this bot, and it does not have
the win10 mixin, and it runs successfully.

Bug: 1166036
R=jam@chromium.org,jochen@chromium.org

Change-Id: Ia99937cc830e4270d37bda309c06e834e0e35676
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626827
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842987}

testing/buildbot/waterfalls.pyl

@@ -1944,7 +1944,6 @@
       'ToTWin64': {
         'mixins': [
           'chrome-tester-service-account',
-          'win10',
           'x86-64',
         ],
         'test_suites': {