Revert "cros: Fix DCHECK for rlz_lib::UpdateExistingAccessPointRlz"

This reverts commit 4da12d2d.

Reason for revert: Flakiness of RlzLibTest.DelayedInitOnlyNoFirstRun
The bug is https://crbug.com/907379

Culprit (70.0% confidence): r609428

Original change's description:
> cros: Fix DCHECK for rlz_lib::UpdateExistingAccessPointRlz
> 
> |UpdateExistingAccessPointRlz| involves file I/O, so it should be
> posted on a task runner that has MayBlock() in its TaskTraits.
> |background_task_runner_| serves this purpose.
> 
> Bug: 906293
> Change-Id: If36215767a0dbcfbad56c0432b16b584f6185376
> Reviewed-on: https://chromium-review.googlesource.com/c/1340607
> Reviewed-by: Roger Tawa <rogerta@chromium.org>
> Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#609428}

TBR=rogerta@chromium.org,wzang@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 906293
Change-Id: I20dbe6c1f73e3599aa30cf8b74512122ad2d0a25
Reviewed-on: https://chromium-review.googlesource.com/c/1345707Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609948}

components/rlz/rlz_tracker.cc

@@ -303,12 +303,7 @@ bool RLZTracker::Init(bool first_run,
       delegate_->IsBrandOrganic(reactivation_brand_)) {
     SYSLOG(INFO) << "RLZ is disabled";
   } else {
-    background_task_runner_->PostTask(
-        FROM_HERE, base::BindOnce(
-                       [](const std::string& brand) {
-                         rlz_lib::UpdateExistingAccessPointRlz(brand);
-                       },
-                       brand_));
+    rlz_lib::UpdateExistingAccessPointRlz(brand_);
   }
 #endif