NaCl: Remove declaration of IrtInit(), which is no longer defined anywhere

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3186
TEST=build

Review URL: https://codereview.chromium.org/157803004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250121 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250121