Remove ChangeLog files from git repository

Remove ChangeLog files from git repository

This information is tracked fully in the git repository, so
no point having the ChangeLog duplicate it, and it interferes
with grepping the repository.

We are going to create the ChangeLogs on the fly when generating
tarballs going forward (with a limited history), since it's still
valuable for tarball consumers to be able to easily see a list of
recent changes.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer-project/-/issues/73

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3521>