Remove support for long gone oldnfs.

The code was calling nmount with an fstype of everything in the program
name after the last '_'. This was there to support mount_nfs being
linked to mount_oldnfs. Support for the link was removed in 2015 with
r281691.

Reviewed by: rmacklem
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10301