md(4): Stop symlinking vn.4 to md.4

We've done the same in the past to the vnconfig.8->mdconfig.8 link in:

eb5f4569819 Remove ancient vnconfig symlink

Reviewed by: bcr, markj, ziaee
Approved by: markj (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27122