freebsd: remove no-op vn_renamepath()

vn_renamepath() is a Solaris-ism that was defined away in the FreeBSD
port. Now that the only use is in the FreeBSD zfs_vnops_os.c, drop it
entierly.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes #14127