tests/lua_loader: remove deprecated test target

The test target is now defined in the common `Makefile.include`.
This removes the warning

    Makefile: warning: overriding recipe for target 'test'
    Makefile.include: warning: ignoring old recipe for target 'test'

tests/lua_loader/Makefile

@@ -11,6 +11,3 @@ ifneq ($(BOARD),native)
 endif
 
 include $(RIOTBASE)/Makefile.include
-
-test:
-	tests/01-run.py