Use get_bot_mapping to generate bot help (Resolves issue #9329)

[DanielBaulig]

Summary

Resolves issue: https://github.com/Rapptz/discord.py/issues/9329

This change makes it so that DefaultHelpCommand uses it's mapping argument (generated by get_bot_mapping()), to generate it's help output. This change does not directly change DefaultHelpCommand behavior and will continue to generate the same output, but allows subclasses to override get_bot_mapping to make alterations to the return value of get_bot_mapping to, for example, add or remove elements.

Checklist

• [x] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes. (Not applicable, actually behavior now matches documentation)
• [x] This PR fixes an issue (https://github.com/Rapptz/discord.py/issues/9329)
• [ ] 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, ...)