makefiles/rust: Remove RIOT_USEMODULE

The latest versions of riot-wrappers and riot-sys do not require it any
more, and the exported data from info-rust is thus much more stable.

makefiles/cargo-targets.inc.mk

@@ -62,7 +62,6 @@ $(CARGO_LIB): $(RIOTBUILD_CONFIG_HEADER_C) $(BUILDDEPS) $(CARGO_COMPILE_COMMANDS
 	@# finally call out to cargo. mind the "+" to pass down make's jobserver.
 	$(Q)+ CC= CFLAGS= CPPFLAGS= CXXFLAGS= \
 		RIOT_COMPILE_COMMANDS_JSON="$(CARGO_COMPILE_COMMANDS)" \
-		RIOT_USEMODULE="$(USEMODULE)" \
 		cargo \
 			build \
 			--target $(RUST_TARGET) \

makefiles/info.inc.mk

@@ -255,4 +255,4 @@ info-rust:
 	@echo "    --target $(RUST_TARGET) --profile $(CARGO_PROFILE)"
 	@echo "and export these environment variables:"
 	@echo "    RIOT_COMPILE_COMMANDS_JSON=\"$(CARGO_COMPILE_COMMANDS)\""
-	@echo "    RIOT_USEMODULE=\"$(USEMODULE)\""
+	@echo "    RIOTBUILD_CONFIG_HEADER_C=\"$(RIOTBUILD_CONFIG_HEADER_C)\""