lib: drop unused protocol headers

- drop unused `http.h` includes.
- drop unused `http1.h` include.
- drop unused `http2.h` includes.
- vssh/ssh.h: drop unused `vssh.h` include.
- urldata.h: drop unused protocol includes.
- url: include `smtp.h` directly.
- rtsp.h: include directly where used.
- imap, smtp: drop redundant include, move another from .h to .c.

Verified with an all non-unity CI run.

Closes #20093

lib/http2.h

@@ -27,7 +27,6 @@
 #include "curl_setup.h"
 
 #ifdef USE_NGHTTP2
-#include "http.h"
 
 /* value for MAX_CONCURRENT_STREAMS we use until we get an updated setting
    from the peer */