fusefs: delete some dead code

The original fusefs GSoC project seems to have envisioned exchanging two
types of messages with FUSE servers. Perhaps vectored and non-vectored?
But in practice only one type has ever been used. Delete the other type.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D27770

(cherry picked from commit 4f4111d2c5ab64591b9e15dab4257d8187458fd1)