Kill aout to elf upgrade path. From an old comment in the makefile:

Kill aout to elf upgrade path.  From an old comment in the makefile:

# If -DWANT_AOUT is specified, a `make world' with OBJFORMAT=elf will
# update the legacy support for aout. This includes all libraries, ld.so
# and boot objects. This part of build should be regarded as
# deprecated and you should _not_ expect to be able to do this past the
# release of 4.0. You have exactly one major release to move entirely
# to elf.

Now that we're past 5.0, these aren't needed at all anymore and it is
well past its freshness date.