fusefs: remove the vfs.fusefs.sync_resize syctl, correctly this time

In r347547 I intended to remove the vfs.fusefs.sync_resize sysctl, leaving
fusefs's behavior as though sync_resize had its default value. But I forgot
that I had already turned off sync_resize in my development system's
/etc/sysctl.conf.

This commit complete removes the optional behavior that was formerly
controlled by sync_resize. There's no need for explicitly calling
FUSE_SETATTR after every FUSE_WRITE that extends a file. The daemon can
infer that the file is being extended. If this sysctl was added as a
workaround for a buggy daemon, there's no clue as to what that daemon may
have been.

Sponsored by: The FreeBSD Foundation