bluetooth: samples: Remove auto phy update from connected CS samples

Unless the PHY is managed by LE power control, the CS procedure phy
needs to match the ACL phy.

Disabling the auto phy updates keeps the ACL on 1M.

The alternative would be to read the current phy by enabling
BT_USER_PHY_UPDATE but this still doesn't work if the phy is changed
after being read.

Signed-off-by: Olivier Lesage <olivier.lesage@nordicsemi.no>