selftests/nolibc: drop call to mrproper target

"mrproper" unnecessarily cleans a lot of files.
kbuild is smart enough to handle changed configurations,
so the cleanup is not necessary and only leads to excessive rebuilds.

Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250123-nolibc-config-v2-3-5701c35995d6@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

Author: t-8ch

tools/testing/selftests/nolibc/Makefile

@@ -263,7 +263,7 @@ initramfs: nolibc-test
 	$(Q)cp nolibc-test initramfs/init
 
 defconfig:
-	$(Q)$(MAKE) -C $(srctree) ARCH=$(ARCH) CC=$(CC) CROSS_COMPILE=$(CROSS_COMPILE) mrproper $(DEFCONFIG)
+	$(Q)$(MAKE) -C $(srctree) ARCH=$(ARCH) CC=$(CC) CROSS_COMPILE=$(CROSS_COMPILE) $(DEFCONFIG)
 	$(Q)if [ -n "$(EXTRACONFIG)" ]; then \
 		$(srctree)/scripts/config --file $(objtree)/.config $(EXTRACONFIG); \
 		$(MAKE) -C $(srctree) ARCH=$(ARCH) CC=$(CC) CROSS_COMPILE=$(CROSS_COMPILE) olddefconfig < /dev/null; \