RFR: 8251861: Remove unused jdk.internal.ref.SoftCleanable and WeakCleanable

[Kiran Sidhartha Ravikumar]

Hi Guys,

Please review the fix to remove jdk.internal.ref.SoftCleanable and WeakCleanable classes as there is no visible usage
of them in JDK. Currently, only PhantomCleanable is used.

More information can be viewed at https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-August/067997.html

Bug: https://bugs.openjdk.java.net/browse/JDK-8251861

Thanks,
Kiran

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Commit messages:
 - 8251861: Remove unused jdk.internal.ref.SoftCleanable and WeakCleanable

Changes: https://git.openjdk.java.net/jdk/pull/616/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=616&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251861
  Stats: 635 lines in 4 files changed: 0 ins; 633 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/616.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/616/head:pull/616

PR: https://git.openjdk.java.net/jdk/pull/616