RFR: 8282040: Remove unnecessary check made obsolete by JDK-8261941

[Calvin Cheung]

Please review this simple change for replacing an `if` block with an `assert`.

Passed CI tiers 1 and 2 testing.

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Commit messages:
 - 8282040: Remove unnecessary check made obsolete by JDK-8261941

Changes: https://git.openjdk.java.net/jdk/pull/8133/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8133&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282040
  Stats: 8 lines in 1 file changed: 0 ins; 7 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8133.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8133/head:pull/8133

PR: https://git.openjdk.java.net/jdk/pull/8133