Remove WebView fallback package handling.

There is no longer a special "fallback package" state for WebView that
means it should not be able to have its enabled/disabled state
controlled by the user. Remove the code used to implement this in
Settings.

Bug: 129470358
Test: verify that WebView's enable/disable button is not greyed out
Change-Id: Ifc0921511a71282a77f239e5ff5955e60fab6e2c

tests/robotests/src/com/android/settings/applications/appinfo/AppButtonsPreferenceControllerTest.java

@@ -119,7 +119,6 @@ public class AppButtonsPreferenceControllerTest {
 
         mController = spy(new AppButtonsPreferenceController(mSettingsActivity, mFragment,
                 mLifecycle, PACKAGE_NAME, mState, REQUEST_UNINSTALL, REQUEST_REMOVE_DEVICE_ADMIN));
-        doReturn(false).when(mController).isFallbackPackage(anyString());
 
         mAppEntry.info = mAppInfo;
         mAppInfo.packageName = PACKAGE_NAME;