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

Makefile.am

@@ -40,7 +40,6 @@ CMAKE_DIST =                                     \
   CMake/FindLibgsasl.cmake                       \
   CMake/FindLibidn2.cmake                        \
   CMake/FindLibpsl.cmake                         \
-  CMake/FindLibrtmp.cmake                        \
   CMake/FindLibssh.cmake                         \
   CMake/FindLibssh2.cmake                        \
   CMake/FindLibuv.cmake                          \