RFR: 8269803: G1: remove unnecessary NoRefDiscovery

[Thomas Schatzl]

On Fri, 2 Jul 2021 11:00:28 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Simple change of removing the unnecessary disable of ref-discovery for CM during young-gc pause.
> 
> Test: hotspot_gc

I think the comment beginning `g1CollectedHeap.hpp:929` should be adapted too.

Looks good otherwise.

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Changes requested by tschatzl (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/4664