RFR: 8356202: Cleanup Source code in String Implementation Classes

[Roger Riggs]

Code cleanup in java.lang.StringLatin1 and StringUTF16.
Remove the `public` qualifier from methods where it is not needed.
Remove an unused toBytes method.
Remove unused codePointXXX methods.
Remove unnecessary casts of primitive types.

Some of these are suggested by IntelliJ, if the advice should not be taken, I'll revert and update the IDE settings.

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Commit messages:
 - Remove unnecessary casts.
 - 8356202: Cleanup public modifiers in String implementation classes

Changes: https://git.openjdk.org/jdk/pull/27609/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27609&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356202
  Stats: 149 lines in 2 files changed: 0 ins; 31 del; 118 mod
  Patch: https://git.openjdk.org/jdk/pull/27609.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27609/head:pull/27609

PR: https://git.openjdk.org/jdk/pull/27609