[ENH] Refactor count/countna to use FExpr

src/core/column/reduce_unary.h

@@ -31,7 +31,7 @@ class ReduceUnary_ColumnImpl : public Virtual_ColumnImpl {
   protected:
     Column col_;
     Groupby gby_;
-    
+
   public:
     ReduceUnary_ColumnImpl(Column &&col, const Groupby& gby, SType stype_out)
       : Virtual_ColumnImpl(gby.size(), stype_out),

src/core/expr/fexpr_minmax.cc

@@ -56,11 +56,9 @@ class FExpr_MinMax : public FExpr_ReduceUnary {
         case SType::INT16:
           return make<int16_t>(std::move(col), gby, is_grouped);
         case SType::INT32:
-        return make<int32_t>(std::move(col), gby, is_grouped);
         case SType::DATE32:
           return make<int32_t>(std::move(col), gby, is_grouped);
         case SType::INT64:
-        return make<int64_t>(std::move(col), gby, is_grouped);
         case SType::TIME64:
           return make<int64_t>(std::move(col), gby, is_grouped);
         case SType::FLOAT32: