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 was used by 2.2% of curl users (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,

scripts/schemetable.c

@@ -46,12 +46,6 @@ static const char *scheme[] = {
   "mqtts",
   "pop3",
   "pop3s",
-  "rtmp",
-  "rtmpt",
-  "rtmpe",
-  "rtmpte",
-  "rtmps",
-  "rtmpts",
   "rtsp",
   "scp",
   "sftp",