RFR: 8357286: (bf) Remove obsolete instanceof checks in CharBuffer.append [v2]

[Brian Burkhalter]

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

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8357286: Correctly handle null CharSequence

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25739/files
  - new: https://git.openjdk.org/jdk/pull/25739/files/6f02b2cd..cda92b2e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25739&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25739&range=00-01

  Stats: 6 lines in 2 files changed: 6 ins; 0 del; 0 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