UnityNuGet usage removal

[bdovaz]

@mayuki Honestly I am disappointed that you have thrown away all my work that took so long to merge (#54) without creating an issue first and at least checking it with me in case I could help before making this drastic change....

With this change you may have broken the integration with OpenUPM and those of us who use UnityNuGet with OpenUPM may start having problems with conflicts and duplications.

[mayuki]

I apologize for potentially disrupting your work. But this change only impacts the contents of the .unitypackage.

Originally, the instructions were to install YetAnotherHttpHandler.Dependencies via UnityNuGet, with no dependencies on YetAnotherHttpHandler itself. And there are currently no plans to change this. YetAnotherHttpHandler.Dependencies for UnityNuGet will remain.

We believe that if using OpenUPM or UnityNuGet, dependencies should be installed from there and should not be resolved by the Package contained in the .unitypackage.
We think it is preferable to use YetAnotherHttpHandler.Dependencies to resolve them from those package registries.