Drop unnecessary compiler guards for memory_sanitizer

We unpoison slots allocated out of the GC, so we don't need to disable
the assertions that read from the memory.

gc/default/default.c

@@ -2119,10 +2119,8 @@ rb_gc_impl_source_location_cstr(int *ptr)
 static inline VALUE
 newobj_init(VALUE klass, VALUE flags, int wb_protected, rb_objspace_t *objspace, VALUE obj)
 {
-#if !__has_feature(memory_sanitizer)
     GC_ASSERT(BUILTIN_TYPE(obj) == T_NONE);
     GC_ASSERT((flags & FL_WB_PROTECTED) == 0);
-#endif
     RBASIC(obj)->flags = flags;
     *((VALUE *)&RBASIC(obj)->klass) = klass;