[webkitpy] Remove unused method from Environment class.

The disable_gcc_smartquotes was used during the build phase of
webkit. This build phase is no longer in the webkitpy so we can
safely remove it.

BUG=none

Review URL: https://codereview.chromium.org/624153004

git-svn-id: svn://svn.chromium.org/blink/trunk@183400 bbb929c8-8fbe-4397-9dbb-9b2b20218538