Remove In-tree C# Bindings (facebook#1302)

Summary:
Pull Request resolved: facebook#1302

These C# bindings were contributed as part of facebook@ff8f17a. They have occasionally been refreshed, but has never really had validation it built continuously, or dedicated maintenance.

There has been a surge of work with facebook#1207 to try to modernize the build for these, but checking with jkoritzinsky I'm not sure either of us have the time to commit to supporting these at the same level as other bindings.

Some well-known projects like Unity had already abandoned this set of bindings for their own. Unity-Technologies/UnityCsReference@016297e#diff-c85198aaac9095a5446ed00b0fba8025072d235b2b69dea8aad85abc64a83e1e

So, as part of the work for an official OSS release, and really trying to define what is deprecated, and what we will try to support, I am removing the in-tree C# bindings from Yoga.

In the past, gaps in Yoga bindings we haven't supported have led to new bindings with dedicated maintainers e.g. [FlexLayout](https://github.com/layoutBox/FlexLayout), [yoga-rs](https://github.com/bschwind/yoga-rs), [yoga-wasm-web](https://github.com/shuding/yoga-wasm-web). My hope is that by removing the C# bindings that we are not supporting, we free up the opportunity for a new version to become the defacto.

Reviewed By: javache

Differential Revision: D46425886

fbshipit-source-id: df964c4d55adf93c4d1e82c104e74ca5ad181612