RFR: 8357286: (bf) Remove obsolete instanceof checks in CharBuffer.append

[Brian Burkhalter]

Remove some instanceof checks which are vestigial now that `CharSequence` itself defines `getChars(int,int,char[],int)`.

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Commit messages:
 - 8357286: (bf) Remove obsolete instanceof checks in CharBuffer.append

Changes: https://git.openjdk.org/jdk/pull/25739/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25739&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357286
  Stats: 48 lines in 2 files changed: 3 ins; 28 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/25739.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25739/head:pull/25739

PR: https://git.openjdk.org/jdk/pull/25739