Correct TRASH() macro usage

TRASH was mapped to TRASH_FREE and was supposed to be used for memory
that should not be accessed anymore, while TRASH_ALLOC() is to be
used for uninitialized but to-be-used memory.

But sometimes TRASH() was used in the latter sense.

Remove TRASH() macro, always use explicit TRASH_ALLOC() or TRASH_FREE().

sql/sql_string.h

@@ -102,7 +102,7 @@ public:
   {
     (void) ptr_arg;
     (void) size;
-    TRASH(ptr_arg, size);
+    TRASH_FREE(ptr_arg, size);
   }
   static void operator delete(void *, MEM_ROOT *)
   { /* never called */ }