[Cleanup] Removed a bunch of superfluous .csproj and .sln files

Most of these files weren't updated in the last >5 years and serve no purpose today, so they only clutter the repository.
Merge pull request #1197 from akoeplinger/remove-csproj

[Cleanup] Removed a bunch of superfluous .csproj and .sln files
Merge pull request #1198 from chriswebb/master

Fixed issue where XmlReflectionImporter unable to handle XmlEnums with colons
Removed [Category("NotWorking")] from PackUriHelperTests.
CompareInvalidTest is still not working because .NET PackUriHelper.ComparePackUri is doing something that uses Uri and throws an exception whereas Mono is not.
Changed Check.PartUriIsValid to throw ArgumentException.
Added argument validations to ComparePackUri an ComparePartUri.
Merge pull request #1199 from esdrubal/packurihelper

PackUriHelper
[mcs] null propagating operator with nested receiver
[jit] Fix enum->int casts in gsharedvt code. Fixes #21893.
[mcs] null propagating operator and extension methods
[runtime] Fix Process.PrivateMemorySize64 etc. on ios. Fixes #21882.
[mcs] null propagating operator and expressions which cannot be null