RFR: 8273929: Remove GzipRandomAccess in heap dump test

[Lin Zang]

The class `GzipRandomAccess` is used to parse heap dump file generated from `jcmd`/`jmap` tools when testing.
It has the limitation that only gzip file which has "blocksize" header field could be sucessfully parsed.
We think this class can be removed for 2 reasons:
1. The gzip heap dump file generated by `jhsdb` command does not contain the "blocksize" header field, so the GzipRandomAccess can not parse the generated file successfully.
2. The `GzipInputStream` could be used instead and then gziped heap dump file generated from both `jcmd`/`jmap` and `jhsdb jmap` could be parsed using same logic.
Options

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Commit messages:
 - 8273929: Remove GzipRandomAccess in heap dump test

Changes: https://git.openjdk.java.net/jdk/pull/5556/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5556&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273929
  Stats: 605 lines in 2 files changed: 20 ins; 578 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5556.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5556/head:pull/5556

PR: https://git.openjdk.java.net/jdk/pull/5556