package/python-meld3: drop package

This project isn't maintained anymore:
https://github.com/Supervisor/meld3/commit/ed4e7b30eeae9e03170f388f5c6fc31aac21bf7d



Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
[Peter: add Config.in.legacy entry]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>