[infra/config] make win7-rel optional in CQ

Now I migrated tests ran on win7-rel to win10_chromium_x64_rel_ng.
And win7-rel does not give us much better additional coverage.
http://shortn/_ZxVQj98HtK
When only win7-rel fails, it almost always only tells us that there
are test flakiness.

I will send PSA when landing this.

Bug: 987847
Change-Id: Ib9b912854e6b43e9985053e8dcb90a98abee65fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1753376
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687155}

docs/infra/cq_builders.md

@@ -70,8 +70,6 @@ These builders must pass before a CL may land.
 
 * [win10_chromium_x64_rel_ng](https://ci.chromium.org/p/chromium/builders/luci.chromium.try/win10_chromium_x64_rel_ng) ([`commit-queue.cfg` entry](https://cs.chromium.org/search/?q=package:%5Echromium$+file:commit-queue.cfg+chromium/try/win10_chromium_x64_rel_ng)) ([matching builders](https://cs.chromium.org/search/?q=file:trybots.py+win10_chromium_x64_rel_ng))
 
-* [win7-rel](https://ci.chromium.org/p/chromium/builders/luci.chromium.try/win7-rel) ([`commit-queue.cfg` entry](https://cs.chromium.org/search/?q=package:%5Echromium$+file:commit-queue.cfg+chromium/try/win7-rel)) ([matching builders](https://cs.chromium.org/search/?q=file:trybots.py+win7-rel))
-
 * [win_chromium_compile_dbg_ng](https://ci.chromium.org/p/chromium/builders/luci.chromium.try/win_chromium_compile_dbg_ng) ([`commit-queue.cfg` entry](https://cs.chromium.org/search/?q=package:%5Echromium$+file:commit-queue.cfg+chromium/try/win_chromium_compile_dbg_ng)) ([matching builders](https://cs.chromium.org/search/?q=file:trybots.py+win_chromium_compile_dbg_ng))
 
 

infra/config/commit-queue.cfg

@@ -140,9 +140,6 @@ config_groups {
       builders {
         name: "chromium/try/win10_chromium_x64_rel_ng"
       }
-      builders {
-        name: "chromium/try/win7-rel"
-      }
       builders {
         name: "chromium/try/win_chromium_compile_dbg_ng"
       }