[ios] Always use symbol name for annotation update

This removes the guard that used to be in place from the time when
UIView backed annotations did not have invisible image placeholders.
Since images are now present for both sprite and UIView backed
annotations, the guard is no longer necessary and removing it allows
for the correct symbol name to be passed to the map object and this
gets rid of `[INFO] {Worker}[Sprite]: Can't find sprite named
'default_marker'` log message.

Author: boundsj

platform/ios/src/MGLMapView.mm

@@ -1768,16 +1768,12 @@ - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(N
         if (annotation == [self annotationWithTag:annotationTag])
         {
             const mbgl::Point<double> point = MGLPointFromLocationCoordinate2D(annotation.coordinate);
-            
-            MGLAnnotationContext &annotationContext = _annotationContextsByAnnotationTag.at(annotationTag);
+
             NSString *symbolName;
-            if (!annotationContext.annotationView)
-            {
-                MGLAnnotationImage *annotationImage = [self imageOfAnnotationWithTag:annotationTag];
-                symbolName = annotationImage.styleIconIdentifier;
-            }
-            
-            _mbglMap->updateAnnotation(annotationTag, mbgl::SymbolAnnotation { point, symbolName.UTF8String ?: "" });
+            MGLAnnotationImage *annotationImage = [self imageOfAnnotationWithTag:annotationTag];
+            symbolName = annotationImage.styleIconIdentifier;
+
+            _mbglMap->updateAnnotation(annotationTag, mbgl::SymbolAnnotation { point, symbolName.UTF8String});
             if (annotationTag == _selectedAnnotationTag)
             {
                 [self deselectAnnotation:annotation animated:YES];