F1: do not assert on unknown F1 message

Previously, in f1ap_decoder.c, we asserted if we did not know a message.
Apart form decoding, we did not do anything in that file.

Additionally, in f1ap_handlers.c, we were also checking for an unknown
message, and printing a warning if a handler is not known.

This commit moves the F1AP message decoding to f1ap_handlers.c, and
deletes f1ap_decoder.c. In summary, we don't assert on an unknown
message, and compile less files. The rest is unchanged.

openair2/F1AP/f1ap_du_interface_management.c

@@ -32,7 +32,6 @@
 
 #include "f1ap_common.h"
 #include "f1ap_encoder.h"
-#include "f1ap_decoder.h"
 #include "f1ap_itti_messaging.h"
 #include "f1ap_du_interface_management.h"
 #include "assertions.h"