samples: bluetooth: central_iso: Remove the CONFIG_BT_SMP=y

Some SoCs in the nRF52 series do not support encrypting and decrypting
isochronous channels packets.

To make it easier to get started with the central_iso sample,
we want to turn of SMP by default. Without this config,
the sample still serves its purpose.
Encrypting the isochronous channel is as simple as
enabling BT_SMP.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>