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

src/libcharon/encoding/payloads/delete_payload.h

@@ -28,11 +28,6 @@ typedef struct delete_payload_t delete_payload_t;
 #include <encoding/payloads/payload.h>
 #include <encoding/payloads/proposal_substructure.h>
 
-/**
- * Length of a delete payload without the SPI in bytes.
- */
-#define DELETE_PAYLOAD_HEADER_LENGTH 8
-
 /**
  * Class representing an IKEv2 DELETE payload.
  *