atm: horizon: Remove some unused functions

Removes some functions that are not used anywhere:
channel_to_vpivci() query_tx_channel_config() rx_disabled_handler()

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: David S. Miller <davem@davemloft.net>