bluetooth: tester: btp: refactors OOB legacy selection

Takes in use IS_ENABLED as preferred way of guarding Kconfig option for
forcing OOB legacy, CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY. It enables
compiling guarded code get warning/errors and not link it if not used. If
def macro is left in btp_handler structure to reduce code size when the
config is not present.

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