[BOLT][NFC][PR] Removed unused singletonSet

Summary:
Remove unused code introduced a while ago (2016), with its use removed
since then.

PR facebookincubator/BOLT#198

Author: Amir Aupov <aaupov@fb.com>

(cherry picked from FBD30376537)

bolt/src/RewriteInstance.cpp

@@ -2980,13 +2980,6 @@ void RewriteInstance::runOptimizationPasses() {
 
 namespace {
 
-template <typename T>
-std::vector<T> singletonSet(T t) {
-  std::vector<T> Vec;
-  Vec.push_back(std::move(t));
-  return Vec;
-}
-
 class BOLTSymbolResolver : public JITSymbolResolver {
   BinaryContext &BC;
   RuntimeDyld &RTDyld;