Remove unused SyncServiceImpl::{Get,Set}DecoupledFromAndroidMasterSync()

No behavior is changed, the methods are unused now. Also add logic to
clear the obsolete pref (should stay around for a year).

Bug: 1107904
Change-Id: Id155da109c716d62defde9a4fd65bfe6f0137573
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3418416
Reviewed-by: Mikel Astiz <[email protected]>
Reviewed-by: Dominic Battré <[email protected]>
Commit-Queue: Victor Vianna <[email protected]>
Auto-Submit: Victor Vianna <[email protected]>
Cr-Commit-Position: refs/heads/main@{#965629}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index ca3573a..6b4119a 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -1617,6 +1617,9 @@
 #if BUILDFLAG(IS_ANDROID)
   // Added 02/2021
   feed::MigrateObsoleteProfilePrefsFeb_2021(profile_prefs);
+
+  // Added 01/2022.
+  syncer::ClearObsoleteSyncDecoupledFromAndroidMasterSync(profile_prefs);
 #endif  // BUILDFLAG(IS_ANDROID)
   syncer::ClearObsoletePassphrasePromptPrefs(profile_prefs);