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

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