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

docs/cmdline-opts/_DESCRIPTION.md

@@ -4,8 +4,8 @@
 
 **curl** is a tool for transferring data from or to a server using URLs. It
 supports these protocols: DICT, FILE, FTP, FTPS, GOPHER, GOPHERS, HTTP, HTTPS,
-IMAP, IMAPS, LDAP, LDAPS, MQTT, MQTTS, POP3, POP3S, RTMP, RTMPS, RTSP, SCP,
-SFTP, SMB, SMBS, SMTP, SMTPS, TELNET, TFTP, WS and WSS.
+IMAP, IMAPS, LDAP, LDAPS, MQTT, MQTTS, POP3, POP3S, RTSP, SCP, SFTP, SMB,
+SMBS, SMTP, SMTPS, TELNET, TFTP, WS and WSS.
 
 curl is powered by libcurl for all transfer-related features. See
 *libcurl(3)* for details.

docs/cmdline-opts/_PROTOCOLS.md

@@ -30,9 +30,6 @@ supported (yet).
 ## POP3(S)
 Downloading from a pop3 server means getting an email. With or without using
 TLS.
-## RTMP(S)
-The **Realtime Messaging Protocol** is primarily used to serve streaming media
-and curl can download it.
 ## RTSP
 curl supports RTSP 1.0 downloads.
 ## SCP