activitypub: remove _misskey_content attribute

As already noted back in https://github.com/misskey-dev/misskey/pull/8787
the intention was to replace the `_misskey_content` attribute with the
ActivityPub-defined `source` property. Misskey and by extension Foundkey
have shipped with the `source` property and the respective parsing for
quite a while so it seems reasonable to remove it now.

Changelog: Removed

packages/backend/src/remote/activitypub/models/note.ts

@@ -207,8 +207,6 @@ export async function createNote(value: string | IObject, resolver: Resolver, si
 	let text: string | null = null;
 	if (note.source?.mediaType === 'text/x.misskeymarkdown' && typeof note.source.content === 'string') {
 		text = note.source.content;
-	} else if (typeof note._misskey_content !== 'undefined') {
-		text = note._misskey_content;
 	} else if (typeof note.content === 'string') {
 		text = fromHtml(note.content, quote?.uri);
 	}

packages/backend/src/remote/activitypub/renderer/index.ts

@@ -35,7 +35,6 @@ export const renderActivity = (x: any): IActivity | null => {
 				value: 'schema:value',
 				// Misskey
 				misskey: 'https://misskey-hub.net/ns#',
-				'_misskey_content': 'misskey:_misskey_content',
 				'_misskey_quote': 'misskey:_misskey_quote',
 				'_misskey_reaction': 'misskey:_misskey_reaction',
 				'_misskey_votes': 'misskey:_misskey_votes',

packages/backend/src/remote/activitypub/renderer/note.ts

@@ -145,7 +145,6 @@ export default async function renderNote(note: Note, dive = true, isTalk = false
 		attributedTo,
 		summary,
 		content,
-		_misskey_content: text,
 		source: {
 			content: text,
 			mediaType: 'text/x.misskeymarkdown',