Target experimental GPU pool on for Win10 NVIDIA GTX 1660 bot

Switching from a single i3-8100 machine to 3 Xeon E-2288G machines.

Also drop driver and OS versions for Linux GTX 1660 bot
to simplify upgrades.

TBR=kbr

Bug: 1042292
Change-Id: I9630d921ae8044ba5ad60e78b8f298a8369d365b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419127
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808577}

testing/buildbot/mixins.pyl

@@ -399,8 +399,8 @@
   'linux_nvidia_gtx_1660_experimental': {
     'swarming': {
       'dimensions': {
-        'gpu': '10de:2184-440.100',
-        'os': 'Ubuntu-18.04',
+        'gpu': '10de:2184',
+        'os': 'Ubuntu',
         'pool': 'chromium.tests.gpu.experimental',
       },
     },
@@ -849,7 +849,7 @@
       'dimensions': {
         'gpu': '10de:2184',
         'os': 'Windows-10',
-        'pool': 'chromium.tests.gpu',
+        'pool': 'chromium.tests.gpu.experimental',
       },
     },
   },