RFR: 8297265: G1: Remove unnecessary null-check in RebuildCodeRootClosure::do_code_blob

[Albert Mingkun Yang]

Simple change of removing an unnecessary check.

Test: hotspot_gc

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Commit messages:
 - g1-code-blob-not-null

Changes: https://git.openjdk.org/jdk/pull/11242/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11242&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297265
  Stats: 5 lines in 1 file changed: 0 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11242.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11242/head:pull/11242

PR: https://git.openjdk.org/jdk/pull/11242