Simplify very outdated core.gyp file list filtering

The gyp build for core does some strange things put in place in ancient
times to fix ancient problems. There's no need nowadays to:

*) list all core svg files in both webcore_files and webcore_svg_files
 only to filter the webcore_files entries with a regex

*) exclude 'opentype', 'cf', etc files from core with a regex

*) exclude platform-specific directories in core - we've moved all
that to Source/platform

These funky rules are hard to maintain in gyp and impossible to do
so in GN, so let's get rid of them.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/354973002

git-svn-id: svn://svn.chromium.org/blink/trunk@177114 bbb929c8-8fbe-4397-9dbb-9b2b20218538