RFR: 8297561: Redundant index check in String.offsetByCodePoints()

[Sergey Tsypanov]

`String.offsetByCodePoints()` delegates to `Character.offsetByCodePoints()` which in turn specifies the same exception thrown under the same conditions and the implementation does exactly the same checks. This means we can remove the check from `String.offsetByCodePoints()` and rely on the one of `Character.offsetByCodePoints()`.

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Commit messages:
 - 8297561: Redundant index check in String.offsetByCodePoints()

Changes: https://git.openjdk.org/jdk/pull/11350/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11350&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297561
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11350.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11350/head:pull/11350

PR: https://git.openjdk.org/jdk/pull/11350