RFR: 8315580: Remove unused java_lang_String::set_value_raw()

[Kim Barrett]

Please review this trivial change to remove the unused function java_lang_String::set_value_raw.

Testing:
mach5 tier1

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Commit messages:
 - remove unused function

Changes: https://git.openjdk.org/jdk/pull/15552/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15552&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315580
  Stats: 5 lines in 2 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15552.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15552/head:pull/15552

PR: https://git.openjdk.org/jdk/pull/15552