tests: Bluetooth: bsim: Fix conn ref

'disconnected' callback always unrefs conn object, but this is only
valid if connection was created in scanx since app owns reference to
that conn object. In case of advx, app does not own any reference so
should not unref.

Basically we do not really need to hold any reference to conn object
in either scanx or advx so just drop reference immediately after conn
is created in scanx, no need to worry about it later.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>