RFR: 8278548: G1: Remove unnecessary check in forward_to_block_containing_addr

[Albert Mingkun Yang]

Simple change of removing an always-false `if`.

Test: I added `assert(false)` inside the `if`, and no failure in tier1-6.

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Commit messages:
 - null-klass-bot

Changes: https://git.openjdk.java.net/jdk/pull/6797/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6797&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278548
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6797.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6797/head:pull/6797

PR: https://git.openjdk.java.net/jdk/pull/6797