RFR: 8292556: Clean up unused Hashtable instantiations

[Coleen Phillimore]

These instantiations in hashtable.cpp are now unused.  Tested with tier1 on Oracle supported platforms.

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Commit messages:
 - 8292556: Clean up unused Hashtable instantiations

Changes: https://git.openjdk.org/jdk/pull/9907/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9907&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292556
  Stats: 22 lines in 1 file changed: 2 ins; 18 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9907.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9907/head:pull/9907

PR: https://git.openjdk.org/jdk/pull/9907