Remove impossible labs feature: sending hidden read receipts

This claims it requires a compatible homeserver, but that does not exist and hasn't for years. Let's just remove the option to stop giving people false hope.

Once notifications are decoupled from read receipts, this sort of thing should be more possible.

src/settings/Settings.ts

@@ -564,13 +564,6 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         // This is a tri-state value, where `null` means "prompt the user".
         default: null,
     },
-    "sendReadReceipts": {
-        supportedLevels: LEVELS_ROOM_SETTINGS,
-        displayName: _td(
-            "Send read receipts for messages (requires compatible homeserver to disable)",
-        ),
-        default: true,
-    },
     "showImages": {
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,
         displayName: _td("Show previews/thumbnails for images"),