remove dev10 related code in Toolset

[JanProvaznik]

ancient code increasing complexity of codebase, dev10 is unsupported

msbuild/src/Build/Definition/Toolset.cs

Line 62 in 41761b8

private const string Dev10OverallInstallKeyRegistryPath = @"Software\Microsoft\DevDiv\vs\Servicing\10.0";

we could increase scope to remove this kind of code from other classes and from other unsupported versions (dev11,12,13, and soon 14)