fusefs: remove the obsolete rename_lock

This lock was included in the original GSoC submission. Its purpose
seems to have been to prevent concurrent FUSE_RENAME operations for the
current mountpoint, as well as to synchronize FUSE_RENAME with
fuse_vnode_setparent. But it's obsolete, now that ef6ea91593e added
mnt_renamelock .

MFC after: 2 weeks
Sponsored by: ConnectWise
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D55231