Remove updateFirstLetter from RenderRubyRun.

This removes the updateFirstLetter override from RenderRubyRun. There are
no test failures when this is removed. It was originally added when the
RubyRun support was added to Blink.

BUG=391288

Review URL: https://codereview.chromium.org/370583003

git-svn-id: svn://svn.chromium.org/blink/trunk@178686 bbb929c8-8fbe-4397-9dbb-9b2b20218538