`DafnySourceAttribute` unconditionally attached to C# assembly

[alex-chew]

When compiling Dafny code to a C#/.NET assembly, the Dafny source is attached to the assembly using a DafnySourceAttribute wrapping a string, and not as just a source comment. This may be important if other Dafny code is to consume the compiled code, but unnecessarily increases the assembly size otherwise. Can we add a flag to omit that attribute?