Remove PartialAppInfo.summary and AppInfo.summary
Remove PartialAppInfo.summary and AppInfo.summary

This is returning an empty String by Discord. and is planned to be fully
removed on v11 of the Discord API. See discord/discord-api-docs#4510.
Harmon758/discord.py,Harmon758/discord.py,rapptz/discord.py,Rapptz/discord.py