Bluetooth: conn: Use delayed work for LE connection timeout

Detaching the timeout from the tx_thread lets us prepare for merging
all TX threads into a single one with the help of k_poll(). There's no
need to define a new delayed work object since the existing connection
parameter update object and connection timeout will never need to run
in parallel. Additionally, but some CONFIG_BLUETOOTH_CENTRAL guards to
void pulling in unnecessary code when Central role is not enabled.

Change-Id: Ia1f222aa052edcecd484a924f2d9a63a3b8fd11f
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>