Remove the "user defined" example in the Reduction Operators section

As far as I can see, this example does not belong here.
Contrary to what it claims, it doesn't define a reduce operator - it
merely defines a completely custom operator that happens to use
square brackets in its symbol.
Nor is is the functionality it implements related to reduce -
rather, it seems to re-implement (part of) the functionality of the
built-in `map` routine.

doc/Language/operators.pod6

@@ -320,14 +320,6 @@ function instead of an operator, provide an additional layer of brackets:
     sub plus { $^a + $^b };
     say [[&plus]] 1, 2, 3;  # 6
 
-They can be defined as list prefix operators or will be generated automatically.
-
-    multi infix:<[~~]> (@c, &test) is looser(&infix:<~~>) {
-        gather for @c { take $_ if test($_) }
-    };
-    my @l = 1, 'a', 2, 'b';
-    say @l [~~] {$^a ~~ Str}; # (a b)
-
 The argument list is iterated without flattening. This means that you can pass
 a nested list to the reducing form of a list infix operator: