Remove extra quoting from the Linux GN platform filter.

This expression is not parsed by the shell, so is not subject to expansion, so
it doesn't need to be quoted. And the quotes were making the regular expression
not match.

R=ilevy@chromium.org, shadi@chromium.org

Review URL: https://codereview.chromium.org/65083002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233683 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233683