RFR: 7505: Cannot reset websocket server port to default

[Alex Macdonald]

This quick change addresses JMC-7505 [[0]](https://bugs.openjdk.java.net/browse/JMC-7505), in which the Preferences menu does not reset the value of the websocket server port.

The fix is to add the missing call to `loadWebsocketPortFromPrefStore()` in the `performDefaults()`.

Before:
![websocket-reset-before](https://user-images.githubusercontent.com/10425301/149178788-f2a12ee2-d6f7-4438-85f2-a56e11c743d6.gif)

After:
![websocket-reset-after](https://user-images.githubusercontent.com/10425301/149178778-f2d707d3-6a7f-4eb7-a603-b2de63aee319.gif)

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Commit messages:
 - 7505: Cannot reset websocket server port to default

Changes: https://git.openjdk.java.net/jmc/pull/353/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=353&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7505
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/353.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/353/head:pull/353

PR: https://git.openjdk.java.net/jmc/pull/353