quartz: Remove unused imageSurface field

imageSurface was used to store a reference to the fallback image
generated by during fallbacks for gradients. The fallback code
has been removed as it is not needed anymore, thus this field can
be removed as well.

src/cairo-quartz-surface.c

@@ -1311,7 +1311,6 @@ typedef struct {
 
     /* Used with DO_IMAGE and DO_TILED_IMAGE */
     CGImageRef image;
-    cairo_surface_t *imageSurface;
     CGRect imageRect;
 
     /* Used with DO_SHADING */
@@ -1438,7 +1437,6 @@ _cairo_quartz_setup_source (cairo_quartz_drawing_state_t *state,
     cairo_status_t status;
 
     state->image = NULL;
-    state->imageSurface = NULL;
     state->shading = NULL;
 
     /* Save before we change the pattern, colorspace, etc. so that
@@ -1596,9 +1594,6 @@ _cairo_quartz_teardown_source (cairo_quartz_drawing_state_t *state,
     if (state->image)
 	CGImageRelease (state->image);
 
-    if (state->imageSurface)
-	cairo_surface_destroy (state->imageSurface);
-
     if (state->shading)
 	CGShadingRelease (state->shading);
 }