rtmp: drop support

- librtmp has no test cases, makes no proper releases and has not had a
  single commit within the last year

- librtmp parses the URL itself and requires non-compliant URLs for this

- we have no RTMP tests

- RTMP is barely used by curl users (2.2% self-identified in the 2025
  survey)

Closes #20673

scripts/cd2nroff

@@ -166,8 +166,6 @@ my %knownprotos = (
     'MQTT' => 1,
     'POP3' => 1,
     'POP3S' => 1,
-    'RTMP' => 1,
-    'RTMPS' => 1,
     'RTSP' => 1,
     'SCP' => 1,
     'SFTP' => 1,