RFR: 8332597: Remove redundant methods from j.l.classfile.ClassReader API

[Adam Sotona]

j.l.classfile.ClassReader instance is exposed in the Class-File API through j.l.classfile.AttributeMapper::readAttribute method only.
ClassReader only purpose is to serve as a tool for reading content of a custom attribute in a user-provided AttribtueMapper.
It contains useful set of low-level class reading methods for user to implement a custom attribute content parser.

However methods ClassReader::thisClassPos and ClassReader::skipAttributeHolder are not necessary for a custom attribute content parsing and so redundant in the API.
Class-File API implementation internally use these methods, however they should not be exposed in the API.

This patch removes the methods from the API.

Please review.

Thanks,
Adam

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Commit messages:
 - 8332597: Remove redundant methods from j.l.classfile.ClassReader API

Changes: https://git.openjdk.org/jdk/pull/19323/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19323&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332597
  Stats: 26 lines in 5 files changed: 0 ins; 21 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/19323.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19323/head:pull/19323

PR: https://git.openjdk.org/jdk/pull/19323