[Disassembler] Delete the VStream parameter of MCDisassembler::getInstruction()

The argument is llvm::null() everywhere except llvm::errs() in
llvm-objdump in -DLLVM_ENABLE_ASSERTIONS=On builds. It is used by no
target but X86 in -DLLVM_ENABLE_ASSERTIONS=On builds.

If we ever have the needs to add verbose log to disassemblers, we can
record log with a member function, instead of passing it around as an
argument.
diff --git a/llvm/tools/llvm-mc/Disassembler.cpp b/llvm/tools/llvm-mc/Disassembler.cpp
index 07fe0d5..e286c0f 100644
--- a/llvm/tools/llvm-mc/Disassembler.cpp
+++ b/llvm/tools/llvm-mc/Disassembler.cpp
@@ -46,8 +46,7 @@
     MCInst Inst;
 
     MCDisassembler::DecodeStatus S;
-    S = DisAsm.getInstruction(Inst, Size, Data.slice(Index), Index,
-                              /*REMOVE*/ nulls(), nulls());
+    S = DisAsm.getInstruction(Inst, Size, Data.slice(Index), Index, nulls());
     switch (S) {
     case MCDisassembler::Fail:
       SM.PrintMessage(SMLoc::getFromPointer(Bytes.second[Index]),