Bluetooth: Controller: Remove conn context NULL check

Remove conn variable NULL check before calling
ull_cp_release_tx() to support conditional compilation of
LLCP_TX_CTRL_BUF_QUEUE_ENABLE variant.

Relates to commit 1ff458ec ("Bluetooth: controller:
llcp: fixing tx buffer queue handling").

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