tools/nolibc: drop s390 clang target override

tools/scripts/Makefile.include now has the same override,
removing the need for the one in the nolibc Makefile.

Drop the superfluous custom override.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://lore.kernel.org/r/20250620-tools-cross-s390-v2-2-ecda886e00e5@linutronix.de


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>