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.

CMakeLists.txt

@@ -449,7 +449,6 @@ add_library(f1ap
             ${F1AP_DIR}/f1ap_cu_task.c
             ${F1AP_DIR}/f1ap_cu_ue_context_management.c
             ${F1AP_DIR}/f1ap_cu_warning_message_transmission.c
-            ${F1AP_DIR}/f1ap_decoder.c
             ${F1AP_DIR}/f1ap_du_interface_management.c
             ${F1AP_DIR}/f1ap_du_paging.c
             ${F1AP_DIR}/f1ap_du_rrc_message_transfer.c