Fix: digittimeout / scheduled hangup issues reported when in immediate mode using newly implemented sched_*_ref functions, correcting an error in the asterisk AST_SCHED_DEL macro. (Reported by Pavel Troller)

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5733 43c1dc8c-776e-4df2-8ef1-6e829299be21