RFR: 8306928: Duplicate variable assignement in jdk.internal.net.http.AuthenticationFilter#getCredentials

[Nizar Benalla]

Picking up a dropped issue please review this simple change.
Passes tier 1-2

TIA

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Commit messages:
 - Removed duplicate assignment in `getCredentials(...)`

Changes: https://git.openjdk.org/jdk/pull/18880/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18880&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306928
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18880.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18880/head:pull/18880

PR: https://git.openjdk.org/jdk/pull/18880