Disable automatic attachment uploading to pastebin in DMs

[wookie184]

Automatic attachment uploading currently has some issues in DMs, one is that it uses clear_reactions

bot/bot/exts/utils/attachment_pastebin_uploader.py

Line 67 in f0e4d45

await message.clear_reactions()

Which raises an error if you attempt to use it in DMs (BOT-3ZK).

IMO it doesn't make much sense in DMs so I think we should just disable it.