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

src/libcharon/encoding/payloads/eap_payload.h

@@ -28,11 +28,6 @@ typedef struct eap_payload_t eap_payload_t;
 #include <encoding/payloads/payload.h>
 #include <sa/authenticators/eap/eap_method.h>
 
-/**
- * Length of a EAP payload without the EAP Message in bytes.
- */
-#define EAP_PAYLOAD_HEADER_LENGTH 4
-
 /**
  * Class representing an IKEv2 EAP payload.
  *