AOSCOS: Revert "FROMLIST: rust: generate a fatal error if BINDGEN_TARGET is undefined"

This reverts "FROMLIST: rust: generate a fatal error if BINDGEN_TARGET is
undefined" as it breaks build on architectures that do not HAVE_RUST.

Link: https://lore.kernel.org/all/6bd26802-e3ce-43e2-b28d-f23e13492458@aosc.io/
Signed-off-by: Mingcong Bai <jeffbai@aosc.io>

Author: MingcongBai

scripts/Makefile.rust

@@ -7,8 +7,4 @@ BINDGEN_TARGET_riscv	:= riscv64-linux-gnu
 BINDGEN_TARGET_um	:= $(BINDGEN_TARGET_$(SUBARCH))
 BINDGEN_TARGET		:= $(BINDGEN_TARGET_$(SRCARCH))
 
-ifeq ($(BINDGEN_TARGET),)
-$(error add '--target=' option to scripts/Makefile.rust)
-else
 export BINDGEN_TARGET
-endif