build: turn off FMTREE by default to prepare for removal

nmtree is derived from fmtree, and has been the default mtree(8) since
6adfbbbf161, a little over a year after its introduction.

fmtree has not seen any substantial work since then, except for build
fixes and runtime issues that were diagnosed in nmtree and backported
because this was still in the tree.

Turn it off by default.

Reviewed by: bdrewery, brooks, cy, emaste
Differential Revision: https://reviews.freebsd.org/D28573