MCSectionMachO: Remove classof

The object file format specific derived classes are used in context like
MCStreamer and MCObjectTargetWriter where the type is statically known.
We don't use isa/dyn_cast and we want to eliminate
MCSection::SectionVariant in the base class.

Author: MaskRay

llvm/include/llvm/MC/MCSectionMachO.h

@@ -87,10 +87,6 @@ class LLVM_ABI MCSectionMachO final : public MCSection {
 
   unsigned getLayoutOrder() const { return LayoutOrder; }
   void setLayoutOrder(unsigned Value) { LayoutOrder = Value; }
-
-  static bool classof(const MCSection *S) {
-    return S->getVariant() == SV_MachO;
-  }
 };
 
 } // end namespace llvm