RFR: 8260391: Remove StringCoding::err

[Claes Redestad]

This patch removes the unused StringCoding::err method, and the associated native code.

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Commit messages:
 - Remove StringCoding::err

Changes: https://git.openjdk.java.net/jdk/pull/2236/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2236&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260391
  Stats: 82 lines in 2 files changed: 0 ins; 82 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2236.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2236/head:pull/2236

PR: https://git.openjdk.java.net/jdk/pull/2236