Remove needless prose in .flat

- Flat returns a Seq with an iterator fed by original stuff, so it solving any
    halting problems is irrelevant.
- Flat used to have a bug where it was failing to propagate .is-lazy in certain
   cases, causing full reification, which might be what the original author hit
   upon, to surmise there are any issues with flattening infinite things.
- Fixes #1202

Author: zoffixznet

doc/Type/Any.pod6

@@ -185,10 +185,6 @@ Defined as:
     method flat(--> Seq:D) is nodal
 
 Interprets the invocant as a list, flattens it, and returns that list.
-Please note that C<.flat> will not solve the
-L<halting problem|https://en.wikipedia.org/wiki/Halting_problem> for you.
-If you flat an infinite list C<.flat> may return that infinite list, eating
-all your RAM in the process.
 
     say ((1, 2), (3)).elems;        # OUTPUT: «2␤»
     say ((1, 2), (3)).flat.elems;   # OUTPUT: «3␤»