RFR: 8329962: Remove CardTable::invalidate

[Albert Mingkun Yang]

Simple converting redundant if-check to assert.

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Commit messages:
 - cardtable-remove-api

Changes: https://git.openjdk.org/jdk/pull/18696/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18696&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329962
  Stats: 13 lines in 3 files changed: 1 ins; 11 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18696.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18696/head:pull/18696

PR: https://git.openjdk.org/jdk/pull/18696