Remove ability of admins to give import userright on zh.wikivoyage

This change removes the ability of administrators on zh.wikivoyage
to grant and revoke the importer right. Transwiki import is going
to be set with change 110876 and it should negate the need of the
importupload permission to be used. zh.wikivoyage has discussed:
//zh.wikivoyage.org/wiki/Wikivoyage:互助客栈#m:Special:Diff.2F7252544_.28Special:Import.29_.E9.97.AE.E9.A2.98

Bug: 61238
Change-Id: I24bb54c8c7cbd6fe3fe1ecd55be9344ee3a157be