torture: Eliminate duplicate #CHECK# from ConfigFragment

The #CHECK# directives that can be present in CFcommon and in the
rcutorture scenario Kconfig files are both copied to ConfigFragment
and grepped out of the two directive files and added to ConfigFragment.
This commit therefore removes the redundant "grep" commands and takes
advantage of the consequent opportunity to simplify redirection.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>