Remove obsolete comment about latin-1 in normalize_encoding

[asottile]

This docstring has drifted since python2:

cpython/Lib/encodings/__init__.py

Line 68 in ca079a3

encoding = encoding.encode('latin-1')

[eamanu]

Hello @asottile

I don't know if is necessary but the bot say that you have to add a new entry on Misc/NEWS. But for me the change is Ok

[asottile]

@eamanu yeah usually for trivial changes like this a core dev will sweep by and add [skip issue] and [skip news] tags (for instance: #7366)