Remove invalid assert from incremental processing

Some time ago "mixed" matches (with both principal and non-principal heads)
were allowed and are handled by Journal. So now principal constraints
can lead to non-principal matches, which renders assert invalid.
(Relevant for MPSCR-62)

reactor/Core/src/jetbrains/mps/logic/reactor/core/internal/ConstraintsProcessing.kt

@@ -235,7 +235,6 @@ internal class ConstraintsProcessing(private var dispatchingFront: Dispatcher.Di
             if (isFront() || !active.isPrincipal()) {
                 matches
             } else {
-                assert( matches.all { it.isPrincipal() } )
                 execQueue.postponeFutureMatches(matches)
             }