Simplify PreParser::ParsePrimaryExpression

* src/preparser.cc (PreParser::ParsePrimaryExpression): No need to check
  keywords for validity as identifiers, as ParseIdentifier (called in
  the fallthrough) will take care of it.

Covered by existing tests.

BUG=

Review URL: https://codereview.chromium.org/12600004
Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13899 ce2b1a6d-e550-0410-aec6-3dcde31c8c00