RFR: 7903895: JMH: Optimize hierarchical padding subclasses

[Aleksey Shipilev]

As [JDK-8345302](https://bugs.openjdk.org/browse/JDK-8345302) shows, generating too much code can choke javac with a small heap size. We need to investigate whether we can cut down on generated code without loss of functionality or performance. One of such opportunities is optimizing the jmhType hierarchy that we use to perform reliable padding.

We do not need one of the subclasses, since it is empty. We also do not need an explicit import for `userType`, since it is already fully qualified in `extends` clause.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jmh/pull/137/files
  Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=137&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903895
  Stats: 19 lines in 1 file changed: 1 ins; 17 del; 1 mod
  Patch: https://git.openjdk.org/jmh/pull/137.diff
  Fetch: git fetch https://git.openjdk.org/jmh.git pull/137/head:pull/137

PR: https://git.openjdk.org/jmh/pull/137