RFR: 2316: Prefer not to specify UTF-8 if it's used by default

[Pavel Rappo]

Please review this clean-up. It's about not specifying UTF-8 encoding when it's the default, such as in `Files.{readString, writeString}`.

The change is very uniform, except for two cases, where specifying the encoding was avoided by slightly more involved refactoring:

  * `JsonWriter:44`
  * `XML:42`

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/skara/pull/1668/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1668&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2316
  Stats: 194 lines in 31 files changed: 0 ins; 36 del; 158 mod
  Patch: https://git.openjdk.org/skara/pull/1668.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1668/head:pull/1668

PR: https://git.openjdk.org/skara/pull/1668