[llvm-reduce] Remove unnecessary includes (NFC)

Identified with clangd.

Author: kazutakahirata

llvm/tools/llvm-reduce/deltas/Delta.cpp

@@ -29,7 +29,6 @@
 #include "llvm/Support/MemoryBufferRef.h"
 #include "llvm/Support/ThreadPool.h"
 #include <fstream>
-#include <set>
 
 using namespace llvm;
 

llvm/tools/llvm-reduce/deltas/Delta.h

@@ -17,7 +17,6 @@
 
 #include "ReducerWorkItem.h"
 #include "llvm/ADT/ArrayRef.h"
-#include "llvm/ADT/ScopeExit.h"
 #include "llvm/Support/raw_ostream.h"
 #include <functional>
 #include <utility>

llvm/tools/llvm-reduce/deltas/ReduceDIMetadata.cpp

@@ -18,8 +18,6 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/InstIterator.h"
-#include <set>
-#include <stack>
 #include <tuple>
 #include <vector>