With `channel_set_cell_handlers()`, two handlers are set in a channel which are for fixed size cell and variable length cell. Turns out that the `channel->var_cell_handler` is never used since all var cell are processed through the channel TLS layer directly: `connection_or_process_cells_from_inbuf()` -> `channel_tls_handle_var_cell()`. Which means we can remove a good 70+ lines of dead code.