Remove stale warning 420 pragmas (dotnet/corefx#35023)

Remove stale warning 420 pragmas (dotnet/corefx#35023)

It used to be that the CS0420 warning ("a reference to a volatile field will not be treated as volatile") would fire when a volatile was used with an Interlocked.* operation.  That warning was unnecessary, as Interlocked.* would itself provide the relevant barriers, and these functions were special cased in Roslyn a long time ago.  But there are still lots of places where pragmas disabling the warning have stuck around.  I'm deleting them.

Commit migrated from https://github.com/dotnet/corefx/commit/8b3446f4e8ad1a7b19c7298e20396629c6e0092e