RFR: 8311581: Remove obsolete code and comments in TestLVT.java

[Daohan Qu]

This is a trivial cleanup patch.

The usage of `testcase.jar` in `LocalVariableTable/TestLVT.java` has been removed in [JDK-8239461](https://bugs.openjdk.org/browse/JDK-8239461). So related code and comments should be removed as well.

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Commit messages:
 - Cleanup

Changes: https://git.openjdk.org/jdk/pull/14784/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14784&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311581
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14784.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14784/head:pull/14784

PR: https://git.openjdk.org/jdk/pull/14784