RFR: 8225235: Unused field defaultIndex in NetworkInterface

[Viktor Klang (Oracle)]

Removes the `defaultIndex`since it is no longer used.

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Commit messages:
 - 8225235: Unused field defaultIndex in NetworkInterface

Changes: https://git.openjdk.org/jdk/pull/10471/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10471&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8225235
  Stats: 10 lines in 2 files changed: 0 ins; 9 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10471.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10471/head:pull/10471

PR: https://git.openjdk.org/jdk/pull/10471