Remove deprecated code

[jbrichau]

Removed the following deprecated code:

• The class JSJsonParser has been deprecated since Seaside 3.1.0 (it was renamed to WAJsonParser)
• The method WARenderContext>>callbackAt: has been deprecated since 3.2.0 (use context callbacks at: instead)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 49.39%. Comparing base (f681d92) to head (69340ee).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1438 +/- ## ======================================= Coverage 49.38% 49.39% ======================================= Files 9080 9078 -2 Lines 80133 80126 -7 ======================================= Hits 39576 39576 + Misses 40557 40550 -7 ```

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[jbrichau]

Requires https://github.com/SeasideSt/Parasol/pull/68 to be merged first to drop the reference to the JsJSONParser