removes channels_builtin when enabling nimPreviewSlimSystem (#20713)

lib/pure/collections/deques.nim

@@ -47,7 +47,6 @@ runnableExamples:
 ## See also
 ## ========
 ## * `lists module <lists.html>`_ for singly and doubly linked lists and rings
-## * `channels module <channels_builtin.html>`_ for inter-thread communication
 
 import std/private/since
 

lib/pure/concurrency/threadpool.nim

@@ -14,7 +14,6 @@
 ## See also
 ## ========
 ## * `threads module <threads.html>`_ for basic thread support
-## * `channels module <channels_builtin.html>`_ for message passing support
 ## * `locks module <locks.html>`_ for locks and condition variables
 ## * `asyncdispatch module <asyncdispatch.html>`_ for asynchronous IO