Fix SAM interception for intercepts other than the first

Fix SAM interception for intercepts other than the first

A country's SAMs launched only in the first interception of a sortie.
That was because ac_intercept() made sam_intercept() delete all SAMs
from the list of available interceptors.  sam_intercept() also deleted
any SAMs out of range.  Don't do that, delete unused SAMs along with
other unused interceptors on return from ac_encounter().