Remove dubious test

S06 currently says:

  Because feeds are defined as lazy pipes, a chain of
  feeds may not begin and end with the same array without
  some kind of eager sequence point. That is, this isn't
  guaranteed to work:

    @DaTa <== grep { $_ % 2 } <== @DaTa;

  [...]

  Conjecture: if the cloning process eagerly duplicates @DaTa,
  it could be forced to work. Not clear if this is desirable,
  since ordinary clones just clone the container, not the value.

'Is not guaranteed to work' sounds rather weak and does not
justify to test for the code given to die.

Also, the code currently lives and the test only succeeded
because @DaTa wasn't visible within 'eval-dies-ok'.

S03-feeds/basic.t

@@ -13,7 +13,7 @@ Tests for the feed operators
     
 =end pod
 
-plan 24;
+plan 23;
 
 {
     my @a = (1, 2);
@@ -64,8 +64,6 @@ plan 24;
     my @data = <1 2 4 5 7 8>;
     my @odds = <1 5 7>;
 
-    eval-dies-ok('@data <== grep {$_ % 2} <== @data', 'a chain of feeds may not begin and end with the same array');
-
     @data = <1 2 4 5 7 8>;
     @data <== grep {$_ % 2} <== eager @data;
     #?rakudo 2 todo 'feeds + eager'