PHP 8.5 error on editing a task without a time zone, removed the unused and broken TaskPicker and tasks/search/json

[yurabakhtin]

https://github.com/humhub/humhub-internal/issues/1355:

Problem

1. For an all-day task with time_zone = NULL, TaskForm::init() sets timeZone to null. TimeZoneDropdownAddition then uses null as an array key, which is an error on PHP 8.5, so the edit form returns 500. On older PHP versions the form silently shows a wrong time zone.
2. tasks/search/json always returns 500. SearchController::actionJson() uses widgets\TaskPicker, an unfinished copy of UserPicker that references Yii::$app->task and a non-existent UserFilter. The model-side TaskPicker and TaskFilter are unfinished stubs too. Nothing in the module or in other modules uses them.

How to reproduce

1. Take an all-day task with time_zone IS NULL, open it and click Edit (PHP 8.5, debug mode on).
2. Open /tasks/search/json?cguid=<space guid> (friendship module enabled).

Fix

1. Fall back to the current form time zone when the task has none.
2. Remove the unused tasks/search/json action, widgets\TaskPicker, models\search\TaskPicker and models\search\TaskFilter.