chan: Remove dead function command_process_var_cell()

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.