m68k: remove BR2_GCC_TARGET_ARCH

Setting BR2_GCC_TARGET_ARCH is possible, but breaks
external toolchain users. m68k/cf defconfigs just
working without it.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

arch/Config.in.m68k

@@ -35,7 +35,3 @@ endchoice
 config BR2_GCC_TARGET_CPU
 	default "68040"		if BR2_m68k_68040
 	default "5208"		if BR2_m68k_cf5208
-
-config BR2_GCC_TARGET_ARCH
-	default "m68k"		if BR2_m68k_m68k
-	default "cf"		if BR2_m68k_cf