RFR: 8322747: StringTable should be AllStatic

[Kim Barrett]

Please review this trivial change to StringTable, making it derive from
AllStatic rather than CHeapObj<mtSymbol>, to match the implementation.

While I was there, also did a little cleanup of related forward and friend
declarations.  StringTableCreateEntry doesn't exist.  Don't need forward
declaration for StringTable right before it's definition.  StringTable doesn't
need to befriend Symbol.

Testing: mach5 tier1

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Commit messages:
 - change base class and tidy

Changes: https://git.openjdk.org/jdk/pull/17193/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17193&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322747
  Stats: 6 lines in 1 file changed: 0 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17193.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17193/head:pull/17193

PR: https://git.openjdk.org/jdk/pull/17193