Don't generate the LICENSE.headless_shell file, since we don't need it, and
the rule for it is annoyingly fragile.

--- a/headless/BUILD.gn
+++ b/headless/BUILD.gn
@@ -980,7 +980,6 @@ executable("headless_shell") {
   deps = [ ":headless_shell_lib" ]
 
   if (proprietary_codecs) {
-    deps += [ ":generate_headless_shell_license_file" ]
   }
 
   if (!headless_use_embedded_resources) {