Col*: Moving code into ColTypeString base

src/columns/ColRecur.cpp

@@ -38,11 +38,6 @@ extern Context context;
 ColumnRecur::ColumnRecur ()
 {
   _name     = "recur";
-
-  // This is 'string', and not 'duration' to force the value to be stored as a
-  // raw duration, so that it can be reevaluated every time.
-  _type     = "string";
-
   _style    = "duration";
   _label    = STRING_COLUMN_LABEL_RECUR;
   _styles   = {"duration", "indicator"};