RFR: 8313962: G1: Refactor G1ConcurrentMark::_num_concurrent_workers

[Albert Mingkun Yang]

Simple cleanup to make this field track the correct number of gc-workers during marking. Some redundant asserts are removed as well. 

Test: hotspot_gc

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Commit messages:
 - g1-conc-threads

Changes: https://git.openjdk.org/jdk/pull/15194/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15194&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313962
  Stats: 16 lines in 1 file changed: 3 ins; 6 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/15194.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15194/head:pull/15194

PR: https://git.openjdk.org/jdk/pull/15194