RFR: 8304144: G1: Remove unnecessary is_survivor check in G1ClearCardTableTask

[Albert Mingkun Yang]

Simple removing redundant check.

Test: tier1-6

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Commit messages:
 - g1-remove-is-survior

Changes: https://git.openjdk.org/jdk/pull/13023/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13023&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304144
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13023.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13023/head:pull/13023

PR: https://git.openjdk.org/jdk/pull/13023