lib/hashtable.c: don't test ->callback in SPL

In SPL, environment callbacks are not supported, so e->callback is
always NULL. Removing this makes the SPL a little smaller (about 400
bytes in my ppc build) with no functional change.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>

lib/hashtable.c

@@ -226,8 +226,10 @@ static int
 do_callback(const struct env_entry *e, const char *name, const char *value,
 	    enum env_op op, int flags)
 {
+#ifndef CONFIG_SPL_BUILD
 	if (e->callback)
 		return e->callback(name, value, op, flags);
+#endif
 	return 0;
 }