Python-telegram-bot: Remove deprecated code

[rahiel]

Through all the versions we've acquired a lot of deprecated code, left intact for backwards compatibility. Here an overview so we won't leave them in forever (between parentheses the first release with the deprecation warning):

Group 1 (#564)

• [x] Non-PEP8 dispatcher methods (v4)
• [x] Non-PEP8 method names in all telegram.ext.*handler.py files (v4)

- [x] Check examples for deprecated code usage

Group 2 (at around 7.0)

• [x] Botan import from utils (v5)
• [x] [telegram.Emoji](https://github.com/python-telegram-bot/python-telegram-bot/blob/5285f63e4a39c171728710e2dd15601e46de9c39/telegram/emoji.py) (v5.1)
• [x] [ReplyKeyboardHide](https://github.com/python-telegram-bot/python-telegram-bot/blob/botapi2.3/telegram/replykeyboardremove.py#L64) (v5.3)
• [x] [edit_message in setGameScore](https://github.com/python-telegram-bot/python-telegram-bot/blob/botapi2.3/telegram/bot.py#L1490) (v5.3)
• [x] Remove usage of deprecated code in tests.

[jh0ker]

Perhaps we could convert this (and potentally other long-running issues like this) into a "Project" so they don't clutter up our Issues page?

[KamranMackey]

@jh0ker That would be a good idea. By the way, there's no need to put Project in quotes. We know what you mean :smile:

Well, I do anyway. xD

[jsmnbom]

It seems you cannot add links to a note in a project, which means that using an issue might still be the best approach (unless we wanna make a issue for each of the code pieces)...
I made a deprecation project to test things out anyway...

[Neurrone]

Should I open a PR that removes all this code at once?

[rahiel]

@Neurrone I've split up the tasks in two groups, with the intention that they will be covered in two PRs. This is because we want to keep the backwards-compatibility for a fair period of time. PRs are welcome, feel free to ask questions on any problems you might encounter and add yourself to AUTHORS.rst if you like.