Remove hack that allowed plug-ins to always take over certain image formats

Remove hack that allowed plug-ins to always take over certain image formats
https://bugs.webkit.org/show_bug.cgi?id=149972

Reviewed by Tim Horton.

This hack was added 8 years ago to allow a certain plug-in to show multi-page TIFF images on uspto.gov.
We now support said TIFFs natively, and the plug-in has been discontinued so it's safe to get rid of this.

* loader/SubframeLoader.cpp:
(WebCore::SubframeLoader::shouldUsePlugin): Deleted.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@190826 268f45cc-cd09-0410-ab3c-d52691b4dbfc