[IRGen] Cond. conformance witness table count isn't needed dynamically.

The count of the number of witness tables was designed to be an
assertion/check that we've hooked up all the infrastructure
correctly. Everything is now implemented, and the assertion has never
triggered, so it can be removed, saving some work.

Fixes rdar://problem/38038928.

lib/IRGen/IRGenModule.cpp

@@ -398,8 +398,6 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
   OpenedErrorTriplePtrTy = OpenedErrorTripleTy->getPointerTo(DefaultAS);
 
   WitnessTablePtrPtrTy = WitnessTablePtrTy->getPointerTo(DefaultAS);
-  WitnessTableSliceTy = createStructType(*this, "swift.witness_table_slice",
-                                         {WitnessTablePtrPtrTy, SizeTy});
 
   InvariantMetadataID = LLVMContext.getMDKindID("invariant.load");
   InvariantNode = llvm::MDNode::get(LLVMContext, {});