Clear aggregation function from field when aggregation is turned off

Problem/Motivation

If a View is not aggregated, the field should not show an aggregation function.

Steps to reproduce

Add two fields.
Enable aggregation for the View.
Configure the aggregation settings for the second field, applying an "average" on the field.
Check the query to see "avg" on the field.
Disable aggregation.
Check the query: "avg" will still be on the field.

Proposed resolution

Add

    $use_aggregation = $this->view->display_handler->useGroupBy();
    $group_type = $use_aggregation ? $this->options['group_type'] : 'group';

    $params = $group_type != 'group' ? ['function' => $group_type] : [];

to the query method of the field.

Remaining tasks

User interface changes

API changes

Data model changes