selftests/nolibc: drop mips32be EXTRACONFIG

kbuild already contains logic to merge predefines snippets into a
defconfig file. For MIPS a snippet for big-endian is already provided.

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

Author: t-8ch

tools/testing/selftests/nolibc/Makefile

@@ -82,7 +82,7 @@ DEFCONFIG_x86        = defconfig
 DEFCONFIG_arm64      = defconfig
 DEFCONFIG_arm        = multi_v7_defconfig
 DEFCONFIG_mips32le   = malta_defconfig
-DEFCONFIG_mips32be   = malta_defconfig
+DEFCONFIG_mips32be   = malta_defconfig generic/eb.config
 DEFCONFIG_ppc        = pmac32_defconfig
 DEFCONFIG_ppc64      = powernv_be_defconfig
 DEFCONFIG_ppc64le    = powernv_defconfig
@@ -93,7 +93,6 @@ DEFCONFIG_s390       = defconfig
 DEFCONFIG_loongarch  = defconfig
 DEFCONFIG            = $(DEFCONFIG_$(XARCH))
 
-EXTRACONFIG_mips32be = -d CONFIG_CPU_LITTLE_ENDIAN -e CONFIG_CPU_BIG_ENDIAN
 EXTRACONFIG           = $(EXTRACONFIG_$(XARCH))
 
 # optional tests to run (default = all)