Makefile: drop unused cpp_cfg macro

Commit e19b0fb4851f (kbuild: generate u-boot.cfg as a byproduct of
include/autoconf.mk) removed the use of the cpp_cfg macro in Makefile,
but forgot to remove its definition.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1053,11 +1053,6 @@ ifndef CONFIG_SYS_UBOOT_START
 CONFIG_SYS_UBOOT_START := 0
 endif
 
-# Create a file containing the configuration options the image was built with
-quiet_cmd_cpp_cfg = CFG     $@
-cmd_cpp_cfg = $(CPP) -Wp,-MD,$(depfile) $(cpp_flags) $(LDPPFLAGS) -ansi \
-	-DDO_DEPS_ONLY -D__ASSEMBLY__ -x assembler-with-cpp -P -dM -E -o $@ $<
-
 # Boards with more complex image requirments can provide an .its source file
 # or a generator script
 ifneq ($(CONFIG_SPL_FIT_SOURCE),"")