[mono] remove runtime support for cominterop

[lambdageek]

It's just dead code now: we never enable mono's built-in COM support on any configuration. If there's a need to support COM with Mono, we should instead focus on the COM interop generator.

In particular, we should delete code like:

  if (MONO_CLASS_IS_IMPORT(...)) {
   // COM stuff
   ...
  }

and

#ifndef DISABLE_COM
// other COM stuff
#endif

[jkoritzinsky]

I'll take a look into this today.