Remove obsolete code from SimpleFontData

Remove unused initCharWidths interface and implementation as well as
unused systemFallback interface from SimpleFontData as no platform uses
or implements either.

BUG=286450
R=pdr@chromium.org

Review URL: https://codereview.chromium.org/617153003

git-svn-id: svn://svn.chromium.org/blink/trunk@183106 bbb929c8-8fbe-4397-9dbb-9b2b20218538