fix(packaging): remove [discord] extra

[shiftinv]

Summary

Removes the [discord] extra, which was used to install the discord-disnake shim. See DisnakeDev/discord-disnake#7.

Installing disnake[discord] has not worked properly with any of the recent versions, given that the shim only works on 2.3 and below. Trying to install this extra on >=2.4 would give you the specified/latest version of disnake and discord-disnake==2.1.2 instead, the only version without a fixed disnake version requirement.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• 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, ...)