RFR: 8306284: G1: Remove assertion in G1ScanHRForRegionClosure::do_claimed_block

[Albert Mingkun Yang]

Simple removing of a superfluous assertion.

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Commit messages:
 - g1-remove-assert

Changes: https://git.openjdk.org/jdk/pull/13509/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13509&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306284
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13509.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13509/head:pull/13509

PR: https://git.openjdk.org/jdk/pull/13509