Revert "USE_MEMPOOL is not defined anywhere"

Well.. then let's remove the ifdef instead of the implementation,
dammit..

This reverts commit 64c6b0ed.

sc/inc/formulacell.hxx

@@ -164,6 +164,10 @@ public:
 
     enum CompareState { NotEqual = 0, EqualInvariant, EqualRelativeRef };
 
+#ifdef USE_MEMPOOL
+    DECL_FIXEDMEMPOOL_NEWDEL( ScFormulaCell )
+#endif
+
     ScAddress       aPos;
 
                     virtual ~ScFormulaCell();

sc/source/core/data/formulacell.cxx

@@ -66,6 +66,10 @@
 
 using namespace formula;
 
+#ifdef USE_MEMPOOL
+IMPL_FIXEDMEMPOOL_NEWDEL( ScFormulaCell )
+#endif
+
 #define DEBUG_CALCULATION 0
 #if DEBUG_CALCULATION
 static ScAddress aDebugCalculationTriggerAddress(1,2,0);    // Sheet1.B3, whatever you like