RFR: 8305095: Update java/net/httpclient/CustomRequestPublisher.java to use new HttpTestServer factory methods

[Daniel Fuchs]

Please find here a trivial change that upgrades the CustomPublisherMethod to use the new HttpTestServer factory methods.

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Commit messages:
 - Removed unused imports
 - 8305095

Changes: https://git.openjdk.org/jdk/pull/13211/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13211&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305095
  Stats: 58 lines in 1 file changed: 0 ins; 38 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/13211.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13211/head:pull/13211

PR: https://git.openjdk.org/jdk/pull/13211