Changes

Avoid removing ASP from all AS during osmo_ss7_as_del_asp()

The osmo_ss7_as_del_asp() -> tcap_asp_down() is weird. In practice it
won't hurt because in IPA we only have 1 ASP per AS, per conceptually is
wrong.
In osmo_ss7_as_del_asp(), you remove ASP from AS. But there you call
asp_down(), which actually removes the ASP from *all* its AS.
So that asp_down() needs to be split and then inside
osmo_ss7_as_del_asp() only remove the ASP from that specific AS.

Change-Id: I37093f5a2fae17a6ef117ac880436bb813bcb2b1