Deprecate WikiRevision::$importer

Deprecate WikiRevision::$importer

This was introduced in 9b3128eb2b654761f21fd4ca1d5a1a4b796dc912
but was not used there.
It also currently looks unused, even when grepping for possible
magic usages of 'importer' and "importer".

We could go ahead and just remove this now, as anything that wants
to set the $importer when the property is removed would still succeed.
But lets adhere to the deprecation policy here.

Change-Id: I5ac16dd87a95be6a3cdd4b16988d37d1d6104749