Android: Remove @MainDex from SystemAccountManagerDelegate.

It isn't necessary and reduces the number of methods in the main dex by
~100.

Bug: 820361
Change-Id: Id08cdf771b255c8ad2de8be46c2dadd69270e029
Reviewed-on: https://chromium-review.googlesource.com/956966Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543463}

components/signin/core/browser/android/java/src/org/chromium/components/signin/SystemAccountManagerDelegate.java

@@ -35,7 +35,6 @@ import org.chromium.base.ContextUtils;
 import org.chromium.base.Log;
 import org.chromium.base.ObserverList;
 import org.chromium.base.ThreadUtils;
-import org.chromium.base.annotations.MainDex;
 import org.chromium.base.library_loader.LibraryLoader;
 import org.chromium.base.metrics.RecordHistogram;
 
@@ -46,7 +45,6 @@ import java.util.concurrent.TimeUnit;
  * Default implementation of {@link AccountManagerDelegate} which delegates all calls to the
  * Android account manager.
  */
-@MainDex
 public class SystemAccountManagerDelegate implements AccountManagerDelegate {
     private final AccountManager mAccountManager;
     private final ObserverList<AccountsChangeObserver> mObservers = new ObserverList<>();