[analyzer][NFC] Remove outdated FIXME comment (#68211)

This trivial commit removes a 13-year-old FIXME comment from
MallocChecker.cpp because it was fixed at least 10 years ago when
`getConjuredHeapSymbolVal()` was added.

clang/lib/StaticAnalyzer/Checkers/MallocChecker.cpp

@@ -1961,13 +1961,10 @@ ProgramStateRef MallocChecker::FreeMemAux(
   // Parameters, locals, statics, globals, and memory returned by
   // __builtin_alloca() shouldn't be freed.
   if (!isa<UnknownSpaceRegion, HeapSpaceRegion>(MS)) {
-    // FIXME: at the time this code was written, malloc() regions were
-    // represented by conjured symbols, which are all in UnknownSpaceRegion.
-    // This means that there isn't actually anything from HeapSpaceRegion
-    // that should be freed, even though we allow it here.
-    // Of course, free() can work on memory allocated outside the current
-    // function, so UnknownSpaceRegion is always a possibility.
-    // False negatives are better than false positives.
+    // Regions returned by malloc() are represented by SymbolicRegion objects
+    // within HeapSpaceRegion. Of course, free() can work on memory allocated
+    // outside the current function, so UnknownSpaceRegion is also a
+    // possibility here.
 
     if (isa<AllocaRegion>(R))
       HandleFreeAlloca(C, ArgVal, ArgExpr->getSourceRange());