Bluetooth: Remove unused function

Remove the function hci_conn_change_link_key() that is not used anywhere.

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: Johan Hedberg <johan.hedberg@intel.com>