Final patch removing setTextAutosizingFontScaleFactor(...)

This patch removes now-dead code in WebSettings.h

This is the last patch of a two-sided change to refactor the code around
setTextAutosizingFontScaleFactor and rename it
setAccessibilityFontScaleFactor.

BUG=320193

Review URL: https://codereview.chromium.org/77663002

git-svn-id: svn://svn.chromium.org/blink/trunk@162338 bbb929c8-8fbe-4397-9dbb-9b2b20218538