[NFC] Remove inaccurate comment

[jfb]

Summary

r271558 moved getManagedStaticMutex's mutex from a function-local
static to using call_once, but left a comment added in r211424. That comment is
now erroneous, remove it.

[chandlerc]

LGTM, thanks for cleaning this up!

lib/Support/ManagedStatic.cpp

 static sys::Mutex *ManagedStaticMutex = nullptr;
 static llvm::once_flag mutex_init_flag;
 
 static void initializeMutex() {
   ManagedStaticMutex = new sys::Mutex();
 }
 
 static sys::Mutex* getManagedStaticMutex() {
-  // We need to use a function local static here, since this can get called
-  // during a static constructor and we need to guarantee that it's initialized
-  // correctly.
   llvm::call_once(mutex_init_flag, initializeMutex);
   return ManagedStaticMutex;
 }
 
 void ManagedStaticBase::RegisterManagedStatic(void *(*Creator)(),
                                               void (*Deleter)(void*)) const {
   assert(Creator);
   if (llvm_is_multithreaded()) {