Revert "[chromium] temporarily disable `luci.buildbucket.backend_alt` for win_optional_gpu_tests_rel"

This reverts commit 0269fc7d00e13e91b5e1df3b29806a8c5ea5650e.

Reason for revert: permission fix worked: verified by https://ci.chromium.org/ui/p/chromium/builders/try.shadow/win_optional_gpu_tests_rel/4/overview. So we could put the builders back to run on backend.

Original change's description:
> [chromium] temporarily disable `luci.buildbucket.backend_alt` for win_optional_gpu_tests_rel
>
> I suspect the task hang issue relates to the swarming -> backend rollout, so temporarily disable it.
>
> Bug: 333068134
> Change-Id: I6f24ca499c09c0d08d2aa0d3537380406d766479
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5444379
> Reviewed-by: Stephanie Kim <kimstephanie@google.com>
> Commit-Queue: Chan Li <chanli@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1285491}

Bug: 333068134
Change-Id: If5ddf03800f93743015ac17b2d51a88a6b43a089
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5448290
Commit-Queue: Stephanie Kim <kimstephanie@google.com>
Auto-Submit: Chan Li <chanli@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Stephanie Kim <kimstephanie@google.com>
Cr-Commit-Position: refs/heads/main@{#1286138}
diff --git a/infra/config/generated/luci/cr-buildbucket.cfg b/infra/config/generated/luci/cr-buildbucket.cfg
index b8fd9ee..507bbeb 100644
--- a/infra/config/generated/luci/cr-buildbucket.cfg
+++ b/infra/config/generated/luci/cr-buildbucket.cfg
@@ -102477,10 +102477,6 @@
         value: 100
       }
       experiments {
-        key: "luci.buildbucket.backend_alt"
-        value: 0
-      }
-      experiments {
         key: "luci.recipes.use_python3"
         value: 100
       }
@@ -105508,10 +105504,6 @@
         value: 100
       }
       experiments {
-        key: "luci.buildbucket.backend_alt"
-        value: 0
-      }
-      experiments {
         key: "luci.recipes.use_python3"
         value: 100
       }
diff --git a/infra/config/subprojects/chromium/try/tryserver.chromium.win.star b/infra/config/subprojects/chromium/try/tryserver.chromium.win.star
index d53f482..85bb4fd 100644
--- a/infra/config/subprojects/chromium/try/tryserver.chromium.win.star
+++ b/infra/config/subprojects/chromium/try/tryserver.chromium.win.star
@@ -113,8 +113,6 @@
     experiments = {
         # crbug/940930
         "chromium.enable_cleandead": 100,
-        # TODO(b/333068134): remove after the bug is fixed.
-        "luci.buildbucket.backend_alt": 0,
     },
     main_list_view = "try",
     reclient_jobs = reclient.jobs.HIGH_JOBS_FOR_CQ,
@@ -499,8 +497,6 @@
     os = os.WINDOWS_DEFAULT,
     # default is 6 in _gpu_optional_tests_builder()
     execution_timeout = 5 * time.hour,
-    # TODO(b/333068134): remove after the bug is fixed.
-    experiments = {"luci.buildbucket.backend_alt": 0},
     main_list_view = "try",
     tryjob = try_.job(
         location_filters = [