[Signin][Android] Remove unused method ProfileDataSource#getProfileDataMap

This CL removes the unused method ProfileDataSource#getProfileDataMap
in the ProfileDataSource interface and updates the interface comment as
there is no requirement to invoke the interface on UI thread now.

Bug: 1136452
Change-Id: Iadbc7021d3acc6b3405cc996428c73ddf6774787
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2620619Reviewed-by: Tanmoy Mollik <triploblastic@chromium.org>
Commit-Queue: Alice Wang <aliceywang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842000}

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

@@ -8,10 +8,8 @@ import android.graphics.Bitmap;
 
 import androidx.annotation.Nullable;
 
-import java.util.Map;
-
 /**
- * Observable source of profile data for accounts on device. Must be used from UI thread only.
+ * Observable source of profile data for accounts on device.
  */
 public interface ProfileDataSource {
     /**
@@ -80,16 +78,6 @@ public interface ProfileDataSource {
         void removeProfileData(String accountEmail);
     }
 
-    /**
-     * Gets ProfileData for all accounts. There must be at least one active observer when this
-     * method is invoked (see {@link #addObserver}).
-     * @return unmodifiable map of ProfileData for all accounts (keyed by account name).
-     * TODO(crbug/1136452): Remove this method after removing the downstream implementation.
-     */
-    default Map<String, ProfileData> getProfileDataMap() {
-        return null;
-    }
-
     /**
      * Gets ProfileData for single account. There must be at least one active observer when this
      * method is invoked (see {@link #addObserver}).