Remove autotools build

Weston 6.0.0 was released with both autotools and Meson build systems. That
should be enough for downstream to migrate to Meson build on their on pace.

Maintaining two build systems is a hassle, keep the one that is easier to work
with and let the other one go.

doc/dozygen/tool*.doxygen.in are not deleted, because they have not been
integrated with Meson yet.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>