doc: fix note about Xen credit scheduler
    
    Commit 6c504d6a added a note to the virsh man page about the
    deprecation of 'cap' and 'weight' settings for the credit
    scheduler. To this day, the default scheduler in Xen is credit
    and it supports setting 'cap' and 'weight'. Remove the deprecation
    notice from the note on the Xen credit scheduler.
    
    Reported-by: Volo M. <vm@vovs.net>