Remove an extraneous include

SymbolWrapper.cpp doesn't use std::optional or llvm::Optional, so this
patch removes the extraneous include.  Note that llvm/ADT/Optional.h
has been deprecated upstream.  This patch ensures that
SymbolWrapper.cpp continues to compile even after the upcoming removal
of Optional.h.

Author: kazutakahirata

compiler/rustc_llvm/llvm-wrapper/SymbolWrapper.cpp

@@ -9,7 +9,6 @@
 
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/Object/ObjectFile.h"
-#include "llvm/ADT/Optional.h"
 
 using namespace llvm;
 using namespace llvm::sys;