misc: Forget about some of the objects to help the GC

After we are done using these objects we can set the
reference to nil. This way the GC can recycle them
more quickly.

src/GSMMOCall.st

@@ -285,7 +285,7 @@ GSMTransaction subclass: GSMMOCall [
     ]
 
     cancel [
-        remoteLeg ifNotNil: [remoteLeg netTerminate].
+        remoteLeg ifNotNil: [remoteLeg netTerminate. remoteLeg := nil].
         ^ super cancel
     ]
 

src/SIPCall.st

@@ -54,6 +54,7 @@ Osmo.SIPCall subclass: SIPMTCall [
 
         newLeg := msc selectRedirectFor: self to: aContact.
         remoteLeg changeRemoteLeg: newLeg.
+        remoteLeg := nil.
     ]
 
     sessionNew [