RFR: 8368024: Remove StringConcatFactory#generateMHInlineCopy

[Shaojin Wen]

With the introduction of [8336856: Efficient hidden class-based string concatenation strategy](https://github.com/openjdk/jdk/pull/20273) , the StringConcatFactory#generateMHInlineCopy method is no longer used by default.

After a year of community use, the InlineHiddenClassStrategy has proven to work well, so I recommend removing the implementation of StringConcatFactory#generateMHInlineCopy.

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Commit messages:
 - clean up StringConcatHelper
 - remove generateMHInlineCopy

Changes: https://git.openjdk.org/jdk/pull/27374/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27374&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368024
  Stats: 783 lines in 5 files changed: 1 ins; 780 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27374.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27374/head:pull/27374

PR: https://git.openjdk.org/jdk/pull/27374