RFR: 8310541: G1: Remove redundant check in G1Policy::need_to_start_conc_mark

[Albert Mingkun Yang]

Simple removing a redundant check, which has already been covered in the early-return part.

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Commit messages:
 - g1-state-redundant

Changes: https://git.openjdk.org/jdk/pull/14588/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14588&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310541
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14588.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14588/head:pull/14588

PR: https://git.openjdk.org/jdk/pull/14588