Drop build-api support

Drop build-api support

It appears the build-api effort at
https://github.com/cgwalters/build-api hasn't really caught on.
systemd appears one of the very few projects actually supporting it.

It does confuse certain tools though. E.g. debhelper by finding a
configure script wrongly assumes this is an autoconf project and thus
needs to be told explicitly that this is in fact a Meson project [1].

Given that Meson is an established build system by now, it appears ok to
drop this compat layer, which will never be fully complete anyway.

[1] https://salsa.debian.org/systemd-team/systemd/-/blob/debian/master/debian/rules?ref_type=heads#L281