Our ::mongo::stdx::variant type used to be a conditional alias, to support different platforms. But as of SERVER-41414 it was no longer conditional, and as of SERVER-77009, it's always std::variant.

Now that ::mongo::stdx::variant and std::variant are always the same, it means code that incorrectly mixes std::variant with ::mongo::stdx::get or vice versa will not be caught in any build variant. So the alias is no longer giving us the benefit of easily switching implementations.

Lets remove the alias and find-replace everything to use std::variant directly.