RFR: 8314321: Remove unused field jdk.internal.util.xml.impl.Attrs.mAttrIdx

[Andrey Turbanov]

A field `char mAttrIdx` in the `jdk.internal.util.xml.impl.Attrs` class is unused and can be removed.

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Commit messages:
 - [PATCH] Remove unused field jdk.internal.util.xml.impl.Attrs.mAttrIdx

Changes: https://git.openjdk.org/jdk/pull/15298/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15298&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314321
  Stats: 7 lines in 1 file changed: 0 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15298.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15298/head:pull/15298

PR: https://git.openjdk.org/jdk/pull/15298