[VarExporter] Remove LazyGhostTrait and LazyProxyTrait in favor of native lazy objects

[alamirault]

Fix #21059

Side question:

For 8.0 branch, what is the process for removing deprecations and versionadded ? Wait new issues from code repository and then remove remaining directives ?

[javiereguiluz]

Antoine, thanks for contributing this.

About your question:

• versionadded can be removed all at once because they are guaranteed (at least they should) to not contain any useful content (just "XXX was introduced in Symfony N.N")
• deprecated directives and deprecated contents should be removed one by one as Symfony code repository creates issues about removed things. Sometimes, removing a deprecation requires small rewords and tweaks in other parts.