Remove some ascii checks in advance of latin-1 release

BUG=

Review URL: https://chromiumcodereview.appspot.com/11880017
Patch from Dan Carney <dcarney@google.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13399 ce2b1a6d-e550-0410-aec6-3dcde31c8c00