refactor!: Remove some unneeded stock strings (#7496)

There are quite some unneeded stock strings; this PR removes some of
them. None of these stock strings were actually set by the UI, or even
have translations in Transifex.
- We don't have AEAP anymore.
- The "I added/removed member" and "I left the group" strings are
anyways not meant to be shown to the user. Also, starting to translate
them now would leak the device language.

BREAKING CHANGE: This can theoretically be a breaking change because a
UI could reference one of the removed stock strings, so I marked it as
breaking just in case.

src/stock_str/stock_str_tests.rs

@@ -75,10 +75,6 @@ async fn test_stock_system_msg_add_member_by_me() {
     let alice_contact_id = Contact::create(&t, "Alice", "alice@example.org")
         .await
         .expect("failed to create contact");
-    assert_eq!(
-        msg_add_member_remote(&t, "alice@example.org").await,
-        "I added member alice@example.org."
-    );
     assert_eq!(
         msg_add_member_local(&t, alice_contact_id, ContactId::SELF).await,
         "You added member Alice."
@@ -91,10 +87,6 @@ async fn test_stock_system_msg_add_member_by_me_with_displayname() {
     let alice_contact_id = Contact::create(&t, "Alice", "alice@example.org")
         .await
         .expect("failed to create contact");
-    assert_eq!(
-        msg_add_member_remote(&t, "alice@example.org").await,
-        "I added member alice@example.org."
-    );
     assert_eq!(
         msg_add_member_local(&t, alice_contact_id, ContactId::SELF).await,
         "You added member Alice."