Remove an unused bit in StringImpl

This code appears to be dead. There aren't any callers anymore.

R=eseidel

Review URL: https://chromiumcodereview.appspot.com/21220005

git-svn-id: svn://svn.chromium.org/blink/trunk@155238 bbb929c8-8fbe-4397-9dbb-9b2b20218538