RFR: 8368968: FloatingDecimal: Clean up unused code

[Johannes Graham]

This is a followup to https://github.com/openjdk/jdk/pull/27118, to clean up resulting unused code.
- remove `getChars`, `isExceptional` and `isNegative` 
- remove `ExceptionalBinaryToASCIIBuffer`; throws `IllegalArgumentException` instead of returning an instance of that.(Caller already asserts non-exceptional arg)

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Commit messages:
 - fix exception
 - clean up unused code

Changes: https://git.openjdk.org/jdk/pull/27581/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27581&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368968
  Stats: 161 lines in 1 file changed: 1 ins; 158 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27581.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27581/head:pull/27581

PR: https://git.openjdk.org/jdk/pull/27581