[MC] flushPendingLabels: revert setAtom change

The setAtom call introduced by e17bc02
was due to my misunderstanding of flushPendingLabels
(see https://discourse.llvm.org/t/mc-removing-aligned-bundling-support/79518).

When evaluating `.quad x-y`,
MCExpr.cpp:AttemptToFoldSymbolOffsetDifference gives different results
at parse time and layout time because the `if (FA->getAtom() ==
FB.getAtom())` condition in isSymbolRefDifferenceFullyResolvedImpl only
works when `setAtom` with a non-null pointer has been called. Calling
setAtom in flushPendingLabels does not help anything.

Author: MaskRay

llvm/lib/MC/MCSection.cpp

@@ -102,12 +102,9 @@ void MCSection::flushPendingLabels() {
   while (!PendingLabels.empty()) {
     PendingLabel& Label = PendingLabels[0];
     switchSubsection(Label.Subsection);
-    const MCSymbol *Atom =
-        CurFragList->Tail ? CurFragList->Tail->getAtom() : nullptr;
     MCFragment *F = new MCDataFragment();
     addFragment(*F);
     F->setParent(this);
-    F->setAtom(Atom);
     flushPendingLabels(F, 0, Label.Subsection);
   }
 }