[make] remove an undefine, unsupported on older gnu makes

It wasn't really that important anyway, was just a general nicety in the
riscv rules.mk

arch/riscv/rules.mk

@@ -179,8 +179,6 @@ else
     $(error SUBARCH not set or set to something unknown)
 endif
 
-undefine RISCV_MARCH_EXTENSIONS
-
 # test to see if -misa-spec=2.2 is a valid switch.
 # misa-spec is added to make sure the compiler picks up the zicsr extension by default.
 MISA_SPEC := $(shell $(TOOLCHAIN_PREFIX)gcc $(ARCH_COMPILEFLAGS) -misa-spec=2.2 -E - < /dev/null > /dev/null 2>1 && echo supported)