Refactor direct references for $wp_rewrite class members

[ikailo]

This patch replaces direct references to class members $wp_rewrite->* with their public functions.

Removes:
$wp_rewrite->flush_rules(), $wp_rewrite->add_permastruct(), $wp_rewrite->add_rewrite_tag()

Replaces with:
flush_rewrite_rules(), add_permastruct(), add_rewrite_tag()

Also removes extraneous $wp_rewrite globals.

Follow-Up to #10912

[duck_]

Patch needs a refresh and it requires #19871 since it tries to use add_rewrite_tag() with 3 parameters.

[duck_]

In [20023]:

Reduce references to the $wp_rewrite global because it's no longer used or a wrapper function can be used instead. Fixes #14546.#14546.