USB: EHCI: remove unused variable in unlink_empty_async()

This patch (as1669) removes the check_unlinks_later flag in ehci-hcd's
unlink_empty_async().  It wasn't being used for anything and should
have been removed in an earlier patch, but I forgot about it.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>