chore(Grid): remove warn from assignTypes

There was a warning when defaulting to using "string" as a type for a
column. It was causing more confusion than the help it was providing so
it's been removed.

Closes #3662

Author: c0bra

src/js/core/factories/Grid.js

@@ -726,7 +726,6 @@ angular.module('ui.grid')
           colDef.type = gridUtil.guessType(self.getCellValue(firstRow, col));
         }
         else {
-          gridUtil.logWarn('Unable to assign type from data, so defaulting to string');
           colDef.type = 'string';
         }
       }