[core] Don't use GL_LINEAR if panning w/o rotation nor pitch

src/mbgl/map/transform_state.cpp

@@ -175,10 +175,6 @@ float TransformState::getPitch() const {
 
 #pragma mark - State
 
-bool TransformState::isChanging() const {
-    return rotating || scaling || panning || gestureInProgress;
-}
-
 bool TransformState::isRotating() const {
     return rotating;
 }