rcutorture: Remove obsolete parameter check from mkinitrd.sh

The mkinitrd.sh script no longer takes an argument, so this commit
therefore removes the code that checks for the parameter being present.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

tools/testing/selftests/rcutorture/bin/mkinitrd.sh

@@ -10,7 +10,6 @@
 D=tools/testing/selftests/rcutorture
 
 # Prerequisite checks
-[ -z "$D" ] && echo >&2 "No argument supplied" && exit 1
 if [ ! -d "$D" ]; then
     echo >&2 "$D does not exist: Malformed kernel source tree?"
     exit 1