Remove shell_window target from WATCHLIST

The class ShellWindow has been renamed to AppWindow, but it hasn't been at the given path for months.  Since no one is really watching it, delete the watch.

BUG=none
TEST=none
R=benwells@chromium.org

Review URL: https://codereview.chromium.org/178993002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253264 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253264