Revert #817 due to high-severity prefix command errors introduced by its changes

[krittick]

Summary

This reverts #817 (and by extension, #826) due to the issues it introduced being higher in severity than the intended fix.

For example, using the prefix-based help command causes this error:

Traceback (most recent call last):
  File "C:\Users\ben\PycharmProjects\cbot\venv3.10\lib\site-packages\discord\ext\commands\core.py", line 179, in wrapped
    ret = await coro(*args, **kwargs)
  File "C:\Users\ben\PycharmProjects\cbot\venv3.10\lib\site-packages\discord\ext\commands\help.py", line 838, in command_callback
    mapping = self.get_bot_mapping()
  File "C:\Users\ben\PycharmProjects\cbot\venv3.10\lib\site-packages\discord\ext\commands\help.py", line 382, in get_bot_mapping
    mapping[None] = [c for c in bot.commands if c.cog is None]
  File "C:\Users\ben\PycharmProjects\cbot\venv3.10\lib\site-packages\discord\ext\commands\core.py", line 1185, in commands
    return set(self.all_commands.values())
TypeError: unhashable type: 'SlashCommand'

Using bot.reload_extension causes this error:

Traceback (most recent call last):
[jishaku-related traceback removed]

  File "<repl>", line 2, in _repl_coroutine
    bot.reload_extension("extensions.ext_libtests")
  File "C:\Users\ben\PycharmProjects\cbot\venv3.10\lib\site-packages\discord\cog.py", line 816, in reload_extension
    self._remove_module_references(lib.__name__)
  File "C:\Users\ben\PycharmProjects\cbot\venv3.10\lib\site-packages\discord\cog.py", line 614, in _remove_module_references
    if cmd.module is not None and _is_submodule(name, cmd.module):
AttributeError: 'SlashCommand' object has no attribute 'module'

There are also reports of other related issues in our discord server, (such as error handlers being broken), but I haven't tested that one myself.

I believe all of these issues are caused by the merging of the app and prefix command dicts and we should revert until we can fully test a more thorough implementation of the intended fix from #817.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)

[plun1331]

I'll do more testing next time