There is an option to delete an attribute type defined in a project, but an attribute type can only be deleted if it is not being used by an issue. 

Prerequisites

• You must have "Change" access rights to the CMS project OR the Administrator role in the server
• An Attribute type can only be deleted if it is not in use, i.e., no values are set for it for any issue in the project

To delete an attribute type that is not in use, go to the project's Setup, and select Attribute Types. Right-click on the attribute type as shown below and select Delete Attribute Type. The attribute type will be removed. 

In the below example, the selected attribute type is in use, so it cannot be deleted. 

Deprecating an Attribute Type

If the attribute type is already in use, instead of deleting it, you can mark it as Deprecated. To do this, right-click on the attribute type and select Edit Attribute Type. Check Deprecated and click OK. 

Remove from Issue Editor

Once you have deprecated the attribute type, you will likely want to exclude it from use so it does not appear in the issue editor. If you do not hide it, it will appear with a warning symbol as shown below, and users can still view and/or edit the values with the proper access rights. 

In the below example, the "Type of documentation" attribute is being used with the Issue Type "Request".

To hide the attribute, under Setup, select Issue Types. 

Select the Issue Type where you want to exclude the attribute type. In the below example, "Request".

Right-click on the attribute type in the Included Attribute Types list and select Exclude. The attribute will no longer appear in the editor for the selected Issue Type. You will need to repeat these steps for each Issue Type where you want to exclude the attribute type.