RFR: 8269803: G1: remove unnecessary NoRefDiscovery [v2]

[Albert Mingkun Yang]

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

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4664/files
  - new: https://git.openjdk.java.net/jdk/pull/4664/files/6e58bbd1..f5ebe866

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4664&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4664&range=00-01

  Stats: 42 lines in 1 file changed: 2 ins; 24 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4664.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4664/head:pull/4664

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