net: remove misleading comment

The allowHalfOpen comment was added in commit 8a3befa0 ("net: Refactor
to use streams2") from 2012 but it wasn't true even then as far as I
can tell: Node.js simply always does a shutdown(2) first.

It is true that streams2 withholds the 'end' event when allowHalfOpen
is true but the comment is about a callback that hangs off the 'finish'
event that is emitted after calling `socket.end()`.

PR-URL: https://github.com/nodejs/node/pull/11573


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>