Bluetooth: Host: Remove conditional stack size for BT_SETTINGS

With BT_SETTINGS enabled, there doesn't seem to (anymore?) be any
substantial overhead in stack consumption:

 0x200020b8 BT RX WQ
        options: 0x0, priority: -8 timeout: -9223372036854775808
        state: pending, entry: 0x5ba69
        stack size 2240, unused 1072, usage 1168 / 2240 (52 %)

Let's just remove the conditional default and fall back to the
non-conditional one which is 1200.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>