Bluetooth: Host: Remove unnecessary `#ifndef`

The functions `le_sc_oob_config_set`, `generate_dhkey` and
`display_passkey` in `smp.c` were only defined when
`CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY` was not defined. This created
issues at build time.

Remove the guard as the code calling those functions is not guarded
itself.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>