Bluetooth: Controller: Remove unneeded members from ull_hdr

Several members of the ull_hdr can be removed, saving 12 bytes
from all ULL instances used (and a bit of code as well)

ticks_active_to_start is always 0 and can be removed completely

ticks_prepare_to_start is always set to
HAL_TICKER_US_TO_TICKS(EVENT_OVERHEAD_XTAL_US), so replace usage
of it by this constant

ticks_preempt_to_start is always set to
HAL_TICKER_US_TO_TICKS(EVENT_OVERHEAD_PREEMPT_MIN_US), so replace
usage of it by this constant

In addition, remove logic handling usage of XON_BITMASK since it
was only used by the long removed legacy LL

Signed-off-by: Troels Nilsson <trnn@demant.com>