feat: do not reveal sender's language in read receipts (#5802)

while adapting strings for the recent change about read receipts,
https://github.com/deltachat/deltachat-core-rust/pull/5712 , it turns
out in discussions eg. at
https://github.com/deltachat/deltachat-android/issues/3179 that
untranslated english for the read receipts seem to be sufficient or even
better:

- do not reveal the sender's language
- unexpected languages are confusing - even if you chat in english, you
may get Chinese read receipts
- many clients do not show the text anyways, iirc, eg. Outlook display
the read receipts in context, and Delta Chat of course as well
- afaik, we're leaving comparable `multipart/report` untranslated as
well (sync, but also webxdc updates are practically english only)
- less code, fewer translations needed :)

deltachat-ffi/deltachat.h

@@ -6650,6 +6650,8 @@ void dc_event_unref(dc_event_t* event);
 /// "Message opened"
 ///
 /// Used in subjects of outgoing read receipts.
+///
+/// @deprecated Deprecated 2024-07-26
 #define DC_STR_READRCPT                   31
 
 /// "The message '%1$s' you sent was displayed on the screen of the recipient."
@@ -6657,7 +6659,7 @@ void dc_event_unref(dc_event_t* event);
 /// Used as message text of outgoing read receipts.
 /// - %1$s will be replaced by the subject of the displayed message
 ///
-/// @deprecated Deprecated 2024-06-23, use DC_STR_READRCPT_MAILBODY2 instead.
+/// @deprecated Deprecated 2024-06-23
 #define DC_STR_READRCPT_MAILBODY          32
 
 /// @deprecated Deprecated, this string is no longer needed.
@@ -7376,11 +7378,6 @@ void dc_event_unref(dc_event_t* event);
 /// Used as info message.
 #define DC_STR_SECUREJOIN_WAIT_TIMEOUT 191
 
-/// "The message is a receipt notification."
-///
-/// Used as message text of outgoing read receipts.
-#define DC_STR_READRCPT_MAILBODY2 192
-
 /// "Contact". Deprecated, currently unused.
 #define DC_STR_CONTACT 200