vcount.9: remove obsolete man page

These functions, vcount() and count_dev(), no longer exist in the source
tree.

Reviewed by: rpokala
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39128

(cherry picked from commit b54391a1f83169f847e5f28e29d1938a154d54e0)