This reverts commit 7f9fcb09e2.
`generator.py` is in the process of being moved to a generic place in
the `gir` repository for reuse across crates. This means `-d` cannot be
passed for our GStreamer-specific `gst-gir-files` directory anymore, and
should be configured from `Gir.toml` instead.
We were already using `gir -d` and especially now that our files are
separated across two directories that are relative to the directory
containing Gir.toml this only becomes cumbersome. Besides `gir` lacks
functionality to normalize the path, leading to ie.
gstreamer-gl/egl/sys/../../../gir-files in the version comment as a
result.
