nfscommon: Clean up the code by removing the vnode_vtype() macro

The vnode_vtype() macro was used to make the code compatible
with Mac OSX, for the Mac OSX port.
For FreeBSD, this macro just obscured the code and, therefore,
use of the macro has been deleted by previous commits.
This commit deletes the, now unused, macro.

This commit should not result in a semantics change.