Bluetooth: host: Refactor use of #ifdef to IS_ENABLED()

Refactor use of #ifdef to IS_ENABLED() pattern for handling cancellation
of outgoing connection attempt. Reduce the amount of indentation by
combining outer if-statements.
Move handling of canceled create connection into a helper function.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>