RFR: 8305500: SystemDictionary::find_placeholder/find_class declared but not used

[Coleen Phillimore]

Removed two function declarations we don't use from SystemDictionary.
Tested tier1 on linux/macosx/windows x84/aarch64

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Commit messages:
 - 8305500: SystemDictionary::find_placeholder/find_class declared but not used

Changes: https://git.openjdk.org/jdk/pull/13313/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13313&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305500
  Stats: 19 lines in 1 file changed: 6 ins; 13 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13313.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13313/head:pull/13313

PR: https://git.openjdk.org/jdk/pull/13313