Use new ApiOpenSearchSuggest hook

With Ie0ab9090, the OpenSearchXml extension is merged into core. The new
hook works identically to OpenSearchXml's hook.

The $wgExtractsExtendOpenSearchXml variable should probably be renamed,
but I'm leaving that for a different patch.

Change-Id: I16024aa22578585873cddba1daa4ca0dc05645e1
Merge "Use new ApiOpenSearchSuggest hook"