Remove some unneeded code noticed while looking at StylePendingImage

Remove some unneeded code noticed while looking at StylePendingImage
https://bugs.webkit.org/show_bug.cgi?id=125618

Reviewed by Andreas Kling.

* css/StyleResolver.cpp:
(WebCore::StyleResolver::loadPendingImage): Removed redundant function calls.

* rendering/RenderImageResource.cpp: Removed unneeded nullImage and
usesImageContainerSize member functions.
(WebCore::RenderImageResource::image): Use Image::nullImage directly instead of
through RenderImageResourceImage::nullImage.

* rendering/RenderImageResource.h: Removed unneeded usesImageContainerSize
and nullImage functions. Also removed unneeded includes.

* rendering/RenderImageResourceStyleImage.h: Removed unneeded
usesImageContainerSize override. Nobody was calling it.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@160478 268f45cc-cd09-0410-ab3c-d52691b4dbfc