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 :)

node/lib/constants.ts

@@ -267,7 +267,6 @@ export enum C {
   DC_STR_REACTED_BY = 177,
   DC_STR_READRCPT = 31,
   DC_STR_READRCPT_MAILBODY = 32,
-  DC_STR_READRCPT_MAILBODY2 = 192,
   DC_STR_REMOVE_MEMBER_BY_OTHER = 131,
   DC_STR_REMOVE_MEMBER_BY_YOU = 130,
   DC_STR_REPLY_NOUN = 90,