refactor: Remove nickname parsing

[Jiralite]

Please describe the changes this PR makes and why it should be merged:

• Misleading information with regards to mentioning users in reference discord/discord-api-docs#4734 (comment)
• discord/discord-api-docs@a8f7638

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[ImRodry]

Should also change this

discord.js/packages/discord.js/src/structures/GuildMember.js

Lines 430 to 432 in ac4bc3a

	toString() {
	return `<@${this.nickname ? '!' : ''}${this.user.id}>`;
	}

[almostSouji]

Discord is removing it, but does it hurt to just keep it in the parsing section for legacy reasons (parsing messages where this was still the case)? I do agree with removing it from #toString, so d.js doesn't send it anymore, but do not see any benefit in removing it from the parsing regex

[vladfrangu]

Agreed with @almostSouji, let's remove it just from toString() (aka make toString() call this.user.toString() :megu:)