Revert "Enable SyncTriggerFullKeystoreMigration by default"

This reverts commit f357a95c.

Reason for revert: rollout is not completed yet and has to be done
using finch.

Original change's description:
> Enable SyncTriggerFullKeystoreMigration by default
>
> Bug: 1094326
> Change-Id: I1cc440f13b92fc825cc749c042e8599fdab793c8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2455447
> Commit-Queue: Mikel Astiz <mastiz@chromium.org>
> Auto-Submit: Maksim Moskvitin <mmoskvitin@google.com>
> Reviewed-by: Mikel Astiz <mastiz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#814665}

TBR=mastiz@chromium.org,mmoskvitin@google.com

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

Bug: 1094326
Change-Id: I4df81e8a74ba25d6cb3ff3c71989e701f4b21b17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2576239Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Cr-Commit-Position: refs/heads/master@{#834668}

components/sync/engine/sync_engine_switches.cc

@@ -21,6 +21,6 @@ const base::Feature kSyncSupportTrustedVaultPassphrase{
     "SyncSupportTrustedVaultPassphrase", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kSyncTriggerFullKeystoreMigration{
-    "SyncTriggerFullKeystoreMigration", base::FEATURE_ENABLED_BY_DEFAULT};
+    "SyncTriggerFullKeystoreMigration", base::FEATURE_DISABLED_BY_DEFAULT};
 
 }  // namespace switches