[MC] Remove a duplicate #include (NFC) (llvm#165507)

Identified with readability-duplicate-include.

Author: kazutakahirata

llvm/lib/MC/MCParser/AsmLexer.cpp

@@ -16,7 +16,6 @@
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/MC/MCAsmInfo.h"
-#include "llvm/MC/MCParser/AsmLexer.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/SMLoc.h"
 #include "llvm/Support/SaveAndRestore.h"