Deprecate WikiRevision::$importer

This was introduced in 9b3128eb2b
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

RELEASE-NOTES-1.29

@@ -265,6 +265,7 @@ changes to languages because of Phabricator reports.
 * Removed 'jquery.arrowSteps' module. (deprecated since 1.28)
 * The 'jquery.autoEllipsis' ResourceLoader module is now deprecated.
 * WikiRevision::$fileIsTemp was deprecated.
+* WikiRevision::$importer was deprecated.
 
 == Compatibility ==
 

includes/import/WikiRevision.php

@@ -31,7 +31,12 @@
  * @ingroup SpecialPage
  */
 class WikiRevision {
-	/** @todo Unused? */
+
+	/**
+	 * @since 1.17
+	 * @deprecated in 1.29. Unused.
+	 * @note Introduced in 9b3128eb2b654761f21fd4ca1d5a1a4b796dc912, unused there, unused now.
+	 */
 	public $importer = null;
 
 	/** @var Title */