Test for compatibility removing setting

[AbrilRBS]

Changelog: Feature: Support compatibility plugin removal of nullable settings.
Docs: conan-io/docs#4278

While investigating for #19027, the suggested approach seems not to be documented, and after talking with the team, it seems like we would be ok with making this a feature, so this PR adds a test to cover the case where

• You have a nullable setting, and you have packages that do not define this setting
• You would like your recipe to depend on these packages, even when your recipe does define the setting
• Adding this to the compatibility plugin is the better option

[memsharded]

I think this is good, and sounds it could be documented.

My concern here is the assymetry with command-line or profile-defined settings.
At the moment, it is not possible to "unset" or to define to Python None a setting that would be defined otherwise, something like:

[settings]
compiler.cppstd=20
....

mypkg/*:compiler.cppstd=@None@

Or something to explicit that nullification, otherwise None is taken literally as a string.
I think this ticket #18968 is about this

[memsharded]

Proof of concept for discussion in #19035