labs: Remove unnecessary parens in randrect

Remove the parens around `rand1` in `randrect.evy` as they are no longer
needed due to a recent change that allows niladic function calls in
expressions without parens.

Since this PR is in this file now, let's make it read nicer. There are
probably other evy files that could have this change - one day we may
find them.

Author: camh-

frontend/lab/samples/ifs/img/randrect.evy

@@ -1,10 +1,10 @@
 fill (hsl 270 100 50 10)
 width 0.2
 for range 60
-    x := (rand1) * 100
-    y := (rand1) * 100
-    s1 := (rand1) * 25 + 2
-    s2 := (rand1) * 25 + 2
+    x := rand1 * 100
+    y := rand1 * 100
+    s1 := rand1 * 25 + 2
+    s2 := rand1 * 25 + 2
 
     move x-s1*0.5 y-s2*0.5
     rect s1 s2