bluetooth: tester: btp: refactors LE secure pairing selection

Takes in use IS_ENABLED as preferred way of guarding Kconfig option for
enabling legacy LE secure pairing, CONFIG_BT_SMP_SC_PAIR_ONLY. It enables
compiling guarded code get warning/errors and not link it if not used.

Signed-off-by: Evgenii Kosenko <Evgenii.Kosenko@silabs.com>