To delete a system attribute in Magento, you must first have to make it user defined.

– Go to phpmyadmin
– Go to your magento installation database
– Go to eav_attribute table
– Browse table with attribute_code ‘YOUR_ATTRIBUTE_CODE’ OR browse the table with the attribute_id of your    attribute (‘your attribute’ means the attribute which you want to remove as system attribute)
– Edit the table row of your attribute
– Find the field „is_user_defined“ and set the value to 1
– save the attribute

Now your attribute no longer remains System Attribute
Now you can delete it from Attribute manager