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.