urldata: remove trailers_state

It's not used anywhere. Remove `data->state.trailers_state` completely.

Closes #20960

lib/urldata.h

@@ -658,13 +658,6 @@ typedef enum {
   EXPIRE_LAST /* not an actual timer, used as a marker only */
 } expire_id;
 
-typedef enum {
-  TRAILERS_NONE,
-  TRAILERS_INITIALIZED,
-  TRAILERS_SENDING,
-  TRAILERS_DONE
-} trailers_state;
-
 /*
  * One instance for each timeout an easy handle can set.
  */
@@ -786,8 +779,6 @@ struct UrlState {
   struct Curl_llist httphdrs; /* received headers */
   struct curl_header headerout[2]; /* for external purposes */
   struct Curl_header_store *prevhead; /* the latest added header */
-  trailers_state trailers_state; /* whether we are sending trailers
-                                    and what stage are we at */
 #endif
 #ifndef CURL_DISABLE_COOKIES
   struct curl_slist *cookielist; /* list of cookie files set by