build-sys: Cleanup handling for trivial-httpd-cmdline

build-sys: Cleanup handling for trivial-httpd-cmdline

This way it's clearer this bit is only about the CLI entrypoint
also living in `ostree trivial-httpd`, not the underlying
`ostree-trivial-httpd` binary that's separate now.

Delete the automake conditional for this, and make the manpage
conditional use `if USE_LIBSOUP` the same way the C build does.

Suggested-by: Jonathan Lebon <jonathan@jlebon.com>