Added an API for deleting unused annotation images’ images. When you nil out the image of an MGLAnnotationImage, MGLMapView deletes the sprite from the style and recreates any annotation associated with the MGLAnnotationImage instance; the MGLAnnotationImage’s falls back to SDK’s default annotation image.
In iosapp, deselecting an annotation resets its image to the default; deselecting it again restores the image.
ref #3185
