sipvoiplink: do not lock callMutex_ from pjsip's callback

This logic was added via 9d9a6b3d,
but this will give potential deadlocks, because pj's group lock
will be locked at this point. So we will have:
group-lock->callMutex_
and other methods like SIPCall::hangup will try to lock:
callMutex_->group-lock
causing a deadlock.

The original issue was adding this line due to a use after free,
however since then we fixed the ref counter of inv, so it should
be a valid fix.

Change-Id: I2fdc236ca63487634ebf2659ecdebb8e0c3234fe
GitLab: #586