gn: Remove inaccurate sentence from README file.

It is used already to generate gyp files and there are plans to replace
gyp by gn.

R=scottmg@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/185213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254379 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254379