RFR: 8261998: Remove unused shared entry support from utilities/hashtable

[Kim Barrett]

Please review this small cleanup in the utilities/hashtable facility.  The
support for "shared" entries is no longer needed or used, so is being deleted.

Testing:
mach5 tier1-4 (some CDS tests are in tier4)

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Commit messages:
 - remove shared entry support

Changes: https://git.openjdk.java.net/jdk/pull/2638/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2638&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261998
  Stats: 28 lines in 3 files changed: 0 ins; 23 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2638.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2638/head:pull/2638

PR: https://git.openjdk.java.net/jdk/pull/2638