Change from Class::method to static::method in WP_List_Table

[okvee]

From this source code https://core.trac.wordpress.org/browser/tags/4.9.1/src/wp-admin/includes/class-wp-list-table.php
At the line 910.

Currently is:

$default = WP_List_Table::get_default_primary_column_name();

Change to:

$default = self::get_default_primary_column_name();

self:: can forward static calls but WP_List_Table:: cannot.
In case that WordPress needs to upgrade requirement to PHP 5.3+ it can change to static:: call.

References:
​http://php.net/manual/en/language.oop5.late-static-bindings.php
​https://stackoverflow.com/questions/4718808/php-can-static-replace-self
​https://stackoverflow.com/questions/3481085/self-vs-classname-inside-static-classname-metods-in-php

[desrosj]

Removing Awaiting Review milestone from closed tickets.