grammar: remove the "assert" construct

`assert e1; e2` is morally equivalent to `if e1 then ⊥ else e2`

grammar/grammar.tex

@@ -20,7 +20,7 @@
   \alt e.a | e.a or e
   \alt p:e | e e
   \alt [ e ... e ] | \{ e = e; ...; e = e; \}
-  \alt with e; e | assert e; e
+  \alt with e; e
   \alt if e then e else e
   \alt let x = e; ...; x = e; in e
   \textcolor{blue}{\alt e :: e | nil}