Remove SerializationInfoExtensions

[greymistcube]

Two reasons:

• Without proper documentation, its exact behavior is not intuitive.
• Doesn't seem to provide much convenience.

All in all, I think the net effect is negative to the whole project. 🙄

[s2quake]

These extension methods are often used in serialization, service providers, etc. I think it's better to not use extension methods than to use them, even if it's inconvenient. But I think it depends on the person who uses it, and since there is quite a bit of code that is used now, it would be nice to see if there is a better way to do it than to get rid of it :)
Alternatively, splitting an assembly like dataset.extensions could be another way to go.

[OnedgeLee]

Seems like there are some conflicts of opinion.
I'll remove GFI tag till decision has been made after discussion.