arc: remove #ifndef CONFIG_AS_CFI_SIGNAL_FRAME

CONFIG_AS_CFI_SIGNAL_FRAME is never defined for ARC.

Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>