build: fix 'make check' when pdwtags is available

Problem introduced in commit 6818cf86.

* src/remote_protocol-structs: Delete unused struct.

src/remote_protocol-structs

@@ -1615,11 +1615,3 @@ struct remote_domain_get_control_info_ret {
         u_int                      details;
         uint64_t                   stateTime;
 };
-struct remote_message_header {
-        u_int                      prog;
-        u_int                      vers;
-        int                        proc;
-        remote_message_type        type;
-        u_int                      serial;
-        remote_message_status      status;
-};