RFR: 8298087: XML Schema Validation reports an required attribute twice via ErrorHandler

[Joe Wang]

Fix a copy n paste error in XMLSchemaValidator. When the patch was applied while updating to Xerces 2.12.2, the original code was not removed, resulting in reporting validation error twice.

-------------

Commit messages:
 - 8298087: XML Schema Validation reports an required attribute twice via ErrorHandler

Changes: https://git.openjdk.org/jdk/pull/13722/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13722&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298087
  Stats: 132 lines in 2 files changed: 126 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13722.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13722/head:pull/13722

PR: https://git.openjdk.org/jdk/pull/13722