Get rid of existential quantifier in invariant-opening rule of logically...

Get rid of existential quantifier in invariant-opening rule of logically atomic triples. This matches what is stated in the Iris 1 paper, and is actually not less expressive.

papers/icfp2021/queue-spec-sc.tex

@@ -76,7 +76,7 @@ This linearization point being atomic, it allows the opening of invariants just
   \infer{%
     \lahoare <x> {\later I \isep P} {e} {\later I \isep Q}
   }{%
-    \knowInv{}{\Exists x. I} \vdash \lahoare <x> {P} {e} {Q}
+    \knowInv{}{I} \vdash \lahoare <x> {P} {e} {Q}
   }
 %
 \end{mathpar}