Bluetooth: Controller: Fix BT_TICKER_EXT BT_TICKER_LOW_LAT dependency

BT_TICKER_EXT is not supported with BT_TICKER_LOW_LAT.
Remove redundant select of BT_TICKER_EXT for
BT_CTLR_ADV_EXT.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>