remove duplication in asynchttpserver examples

[timotheecour]

fixes bug 2 in #16506 (comment)

followup after #16580
can test server with:
nim doc -r --doccmd:-d:nimAsynchttpserverEnableTest lib/pure/asynchttpserver.nim

[narimiran]

So, instead of some code duplication, we now have a runnable example which most likely will never be run.

People found the previous bug just by copy-pasting and running the example. If they do the same now, nothing will happen because of if (!) defined, with no mention in the example when this condition is true, i.e. how to enable it.

[timotheecour]

--docCmd is documented already; in any case I've added a comment to show how to run this; no copy paste needed. And anyone with more than a few hours of experience with nim should know how to pass -d:foo on cmdline. Much better than duplication anyways.

[Araq]

Why is this an if defined? What's wrong wtih when defined?

[timotheecour]

@Araq

Why is this an if defined? What's wrong wtih when defined?

it's intentional, otherwise the code is never semchecked and makes the runnableExamples much less useful for CI; it's a good pattern IMO, to use whenever we want the example to keep compiling but not run by default unless a flag is specified

case in point: see nim-lang/fusion#67 (just today), where a bug was found thanks to if defined(fusionJsFormdataTests): being used instead of when defined(fusionJsFormdataTests): in runnableExamples

[ghost]

IMO that example will be very confusing for people who read asynchttpserver's docs. I understand that it reduces code duplication, but generally, the examples in stdlib that we have don't need to be run from the module itself.