Remove an obsolete comment

This is a follow up CL to our previous CL [1], where we forgot to
remove an obsolete comment, which was dded in our previous attempt to
tackle IME focus handling in InputMethodManager [2].  As the line
mentioned in that CL is now gone, there is no reason to keep that
stale comment.

There should be no behavior change in this CL.

 [1]: Ib455704fe1e9d243f93190a84f230210dbceac2a
      970d9d2e0c
 [2]: I219394178e4172bc47864297f1418e677dba25e5
      5f05965f54

Bug: 141738570
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: Iee92a7eb4dfcfb059b0e1fc105f78dd9bdecbbfc

core/java/android/view/inputmethod/InputMethodManager.java

@@ -664,8 +664,6 @@ public final class InputMethodManager {
          */
         @Override
         public void setCurrentRootView(ViewRootImpl rootView) {
-            // If the mCurRootView is losing window focus, release the strong reference to it
-            // so as not to prevent it from being garbage-collected.
             if (mWindowFocusGainFuture != null) {
                 mWindowFocusGainFuture.cancel(false /* mayInterruptIfRunning */);
                 mWindowFocusGainFuture = null;