Back to the roots, remove C# and Python

[a4z]

The C# and Python parts are a maintenance burden.

I plan to release a new major version without those two languages.
If anyone wants to keep them alive in a branch of the current major version, welcome.

Otherwise, this project should focus again on its core components.
Objective-C/Swift and Java/Kotlin, to have some chance of ongoing maintenance.

Edit:
PR #177
More context: #179
this is the final scala version for 2.x, next update will then target scala3

[a4z]

library adoption: cross-language-cpp/djinni-support-lib#82