mountd(8): parsecred(): Remove comment on non-existent bug

'name' can never be NULL, as it is equal to 'names' and 'namelist',
which was passed by do_cred() only if the actual parameter wasn't NULL.

Reviewed by: rmacklem
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47014