Stop warn about failure to get a macro when preparing a XDOM

The problem is that the root cause for this 99% of the time is just that the macro does not exist, and there is no reason to flood the general log with something that minor.

We can enable it back when we introduce the concept of log associated to a content execution.

It was not too visible so far but with XWIKI-21219 I'm afraid it will happen way too often.