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/sendf.h

@@ -26,8 +26,6 @@
 
 #include "curl_setup.h"
 
-#include "curl_trc.h"
-
 /**
  * Type of data that is being written to the client (application)
  * - data written can be either BODY or META data
@@ -55,6 +53,7 @@
 /* Forward declarations */
 struct Curl_creader;
 struct Curl_cwriter;
+struct Curl_easy;
 
 /**
  * Write `len` bytes at `prt` to the client. `type` indicates what