[MC] flushPendingLabels: revert setAtom change

The setAtom call introduced by e17bc023
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.