Remove SVGExternalResourcesRequired.h

r164692 removed SVGExternalResourcesRequired.cpp and .idl, but left the
header file. Remove it.

R=pdr@chromium.org,schenney@chromium.org,fmalita@chromium.org

Review URL: https://codereview.chromium.org/159893011

git-svn-id: svn://svn.chromium.org/blink/trunk@167020 bbb929c8-8fbe-4397-9dbb-9b2b20218538