Force non-public taxonomies to have a query_var of `false`.

[35333] implemented `public=false` for taxonomies. The implementation prevented
non-public taxonomies from having their archives accessed via query_var during
a normal request. But it didn't prevent non-public taxonomies from registering
their query vars in the `$wp_taxonomies` global. The latter implementation
details causes problems specifically when a taxonomy is registered with
`query_var=true`; for public taxonomies, `register_taxonomy()` translates this
into a query_var equivalent to the taxonomy name, but in the case of non-public
taxonomies, the query_var was set to the boolean itself. The boolean then
causes problems when using non-strict comparison to filter taxonomy objects by
query_var, as when using `get_taxonomies()`.

This changeset addresses the issue by forcing the query_var property of
non-public taxonomies to `false`.

Fixes #35089.
Built from https://develop.svn.wordpress.org/trunk@36108


git-svn-id: https://core.svn.wordpress.org/trunk@36073 1a063a9b-81f0-0310-95a4-ce76da25c4cd