Remove actually_private::T

[Mingun]

This sounds like a change we could do on master and consider in isolation from this PR

As posted by @oli-obk in #2608 (comment), this PR removes actually_private::T type from signature of the hidden Deserializer::__deserialize_content method, like Visitor::__private_visit_untagged_option does not have such guard:

serde/serde/src/de/mod.rs

Lines 1679 to 1687 in 2130ba5

	// Used when deserializing a flattened Option field. Not public API.
	#[doc(hidden)]
	fn __private_visit_untagged_option<D>(self, _: D) -> Result<Self::Value, ()>
	where
	D: Deserializer<'de>,
	{
	Err(())
	}
	}

This guard just makes life harder in some cases without real benefits. If you use private API you already understand consequences and them can be totally acceptable, especially if you depend on the specific version

[oli-obk]

Thanks!