Coding Standards: Rename abbreviated variables in WP_Query.

Renames the locally scoped references to `WP_Query::$query_vars` from `$q`/`$qv` to `$query_vars` for clarity per the coding standards' [https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/#naming-conventions naming conventions]:

> Don’t abbreviate variable names unnecessarily; let the code be unambiguous and self-documenting.

See #63168.


git-svn-id: https://develop.svn.wordpress.org/trunk@60927 602fd350-edb4-49c9-b593-d223f7449a82

Author: peterwilsoncc