lib: drop, or replace sendf.h with curl_trc.h where possible

- replace `sendf.h` with `curl_trc.h` where it was included just for it.
- drop unused `curl_trc.h` includes.
- easy: delete obsolete comment about `send.h` include reason.

Also:
- move out `curl_trc.h` include from `sendf.h` and include it directly
  in users, where not done already. To flatten the include tree and
  to less rely on indirect includes.
- stop including `sendf.h` from other headers, replace it with forward
  declaration of `Curl_easy`, as done already elsewhere.

Verified with an all non-unity CI run.

Closes #20061

lib/connect.c

@@ -51,7 +51,7 @@
 #endif
 
 #include "urldata.h"
-#include "sendf.h"
+#include "curl_trc.h"
 #include "strerror.h"
 #include "cfilters.h"
 #include "connect.h"