Remove unneeded public MenuBlock::setMenuParentFormSelector() method

Problem/Motivation

At #3115436: Use proper dependency injection and protect menu_block from changes to the SystemMenuBlock constructor we added a public setMenuParentFormSelector() method to the MenuBlock plugin.

https://www.previousnext.com.au/blog/safely-extending-drupal-8-plugin-cl... for background.

However, we don't actually need that. ;) As @alexpott pointed out with the snippet at https://3v4l.org/KSYat -- static factory methods can access protected members on their newly created instances, without needing public methods.

See patch and interdiff at #2809699-159: Add configuration options for dynamic block titles for a working example of a similar dependency injection on this plugin where we don't need to introduce a new public setter.

Proposed resolution

Remove MenuBlock::setMenuParentFormSelector() and set $instance->menuParentFormSelector directly in MenuBlock::create().

Remaining tasks

1. Wait for #2809699: Add configuration options for dynamic block titles to land, since any patch here will conflict with that, and I don't want to further delay that issue.
2. Decide the BC implications of this change. :/ Sadly, we didn't realize this at #3115436 and already shipped 8.x-1.6 with the public setter. So in theory, someone could be using that already. But I say screw 'em. No one has any business mucking with this stuff. It was a freshly added piece of the public API, it's only been out for a week or so. But maybe we should do the whole deprecation hoop jumping and only remove this in a forthcoming 2.x branch.
3. Patch it.
4. Reviews.
5. RTBC.
6. Commit.

User interface changes

None.

API changes

Removing a piece of public API on the MenuBlock plugin that shouldn't have been added in the first place.

Data model changes

None.

Release notes snippet

TBD. Depends on what we decide above about BC stuff. :/