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

tests/data/test1627

@@ -18,7 +18,7 @@ Curl_get_scheme unit test
 
 <verify>
 <stdout mode="text">
-205 invokes
+199 invokes
 </stdout>
 </verify>
 </testcase>