Remove the `filter` parameter in the Post Controller

[websupporter]

As discussed in ​https://github.com/WP-API/WP-API/issues/2799 the team decided to drop support for the filter parameter. It introduced an "inconsistent mess" and "exposing WP_Query through filter has and will continue to be difficult to support."

[websupporter]

Will update the patch. Lines 169-170 need to be included.

[websupporter]

Our calculation of the max. pages needed posts_per_page to be set in our $query_args. In order to overwrite filter we did so before. When I deleted the filter stuff, I also deleted this, which produced a bug.

Since we always make sure, we add stuff only, when it is given in the schema like

<?php
if ( isset( $registered['per_page'] ) ) {
    $args['posts_per_page'] = $request['per_page'];
}

I thought it might not be the best to rely on posts_per_page is set. In 38378.2.diff I use the $posts_query->query_vars instead:
$max_pages = ceil( $total_posts / (int) $posts_query->query_vars['posts_per_page'] );

[websupporter]

but of course we need to set the posts per page if given...

[kadamwhite]

Technically if somebody has de-registered per_page as a valid parameter, they're probably extending this to write their own endpoint and have voided their pagination warrantee in the process. I'm not sure it makes sense to me to special-case this, when so many other properties that the API depends upon for proper functioning are whitelisted in the same general way using $registered. I would defer to @rmccue and @joehoyle on this, though.

[rachelbaker]

I started working on this earlier, and am finishing up with some test fixes. I will have this committed soon.

[rachelbaker]

In 38968:

REST API: Remove experimental filter wrapper parameter from the Posts Controller class.

Hiding WP_Query params under the filter key (instead of allowing them to be top-level params) was one of our biggest complaints from users of v1 of our REST API. This walks back the re-introduction of the filter param during Beta 15, which introduced an "inconsistent mess" and "exposing WP_Query through filter has and will continue to be difficult to support." See ​https://github.com/WP-API/WP-API/issues/2799.

Props websupporter, rachelbaker.
Fixes #38378.