The `lgpl-docs` and documentation embedding step is now solely invoked
from `./gir/generator.py` in the CI, and does not need the embed/purge
build features anymore.
The `lgpl-docs` crate and all precompiled (easy to get outdated!)
docmentation files are removed in favour of being generated at runtime
by the improved generator, both during local development as well as in
the CI.
This crate was added right around the same date the conversion to the
`lgpl_docs` crate took place in 801998c7 ("Generate documentation from
the docs crate directly") and hence likely missed out.
Fixes: 4120ded4 ("Add gstreamer-controller")
