Add a payload.get_header_length() method, remove header length definitions

src/libcharon/encoding/payloads/ts_payload.h

@@ -30,11 +30,6 @@ typedef struct ts_payload_t ts_payload_t;
 #include <encoding/payloads/payload.h>
 #include <encoding/payloads/traffic_selector_substructure.h>
 
-/**
- * Length of a TS payload without the Traffic selectors.
- */
-#define TS_PAYLOAD_HEADER_LENGTH 8
-
 /**
  * Class representing an IKEv2 TS payload.
  *