Remove mention of supported platforms from Microsoft.CodeAnalysis.Compilers (#18692)

Author: agocke

src/NuGet/Microsoft.CodeAnalysis.Compilers.nuspec

@@ -5,11 +5,6 @@
     <summary>.NET Compiler Platform ("Roslyn")</summary>
     <description>
       .NET Compiler Platform ("Roslyn"). Install this package to get both C# and Visual Basic support. Install either of the dependencies directly to get one of the languages separately.
-
-Supported Platforms:
-- .NET Framework 4.5
-- Windows 8
-- Portable Class Libraries
     </description>
     <dependencies>
       <dependency id="Microsoft.CodeAnalysis.CSharp" version="[$version$]" />