Delete unreachable default clauses. (#561)

The Dart analyzer will soon be changed so that if the `default` clause
of a `switch` statement is determined to be unreachable by the
exhaustiveness checker, a new warning of type
`unreachable_switch_default` will be issued. This parallels the behavior
of the existing `unreachable_switch_case` warning, which is issued
whenever a `case` clause of a `switch` statement is determined to be
unreachable. For details see
dart-lang/sdk#54575.

This PR deletes unreachable `default` clauses from `photo_view` now, to
avoid a spurious warning when the analyzer change lands.

lib/photo_view.dart

@@ -585,8 +585,6 @@ PhotoViewScaleState defaultScaleStateCycle(PhotoViewScaleState actual) {
     case PhotoViewScaleState.zoomedIn:
     case PhotoViewScaleState.zoomedOut:
       return PhotoViewScaleState.initial;
-    default:
-      return PhotoViewScaleState.initial;
   }
 }
 

lib/src/utils/photo_view_utils.dart

@@ -21,9 +21,6 @@ double getScaleForScaleState(
           scaleBoundaries);
     case PhotoViewScaleState.originalSize:
       return _clampSize(1.0, scaleBoundaries);
-    // Will never be reached
-    default:
-      return 0;
   }
 }