The F1AP message encoders and decoders should do only that, encoding and
decoding messages. For the actual logic handling, including set up and
teardown of bearers, we should use the handlers.

In this commit, delete GTP tunnel handling from encoders/decoders. For
the CU, this happens already in the CU-UP/E1 handlers in
cucp_cuup_handler.c.  For the DU, add it in the handlers in
mac_rrc_dl_handler.c.