Ceremonially delete the Python prototype!

Ceremonially delete the Python prototype!

We've reached feature parity, although I haven't been running the Rust
version live for more than a few hours so far, so there's still a
possibility of finding a bug the old version lacked.

But it's now actually awkward to have it in the same directory - the
two clients look similar enough that I can accidentally run
'./mastodonochrome' in place of './target/debug/mastodonochrome'
during testing, and then be puzzled when it doesn't behave _quite_ the
same.

So, if I need it in an emergency, I can always recover it from the git
log.