Remove use of WP_HTML_Tag_Processor in navigation-submenu block

[aristath]

What?

Following the discussion on #47349, this PR removes the use of the WP_HTML_Tag_Processor object in the navigation-submenu block.
This was introduced in #40778 and can be replaced by a simple str_replace call.

Why?

See #47349 for details

How?

Replacing the call to WP_HTML_Tag_Processor with an str_replace() call. It's simple, effective, and it works.

[ockham]

Thank you very much, Ari!

Given the latest conversation on #47349, there's a chance that we'll end up merging WP_HTML_Tag_Processor in Core after all 😅 I'll reply there shortly; let's maybe hold off on this PR for now 😊

[ockham]

Update: We've discussed WP_HTML_Tag_Processor once more, and we've decided to include it in 6.2 after all 😊 This means we won't have to remove the dependency, and we can close this PR.

Thanks again, @aristath -- and sorry for the noise! 😅