Remove GzipRandomAccess in heap dump test

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:

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.
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.
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