Salesforce picklist hide none value

Just go to the Picklist definition on the custom object and select Reorder and it allows you to set a Default value.

Build>Create>Objects>My Object>Custom Fields

Then on the page layout make the picklist required so it will not permit a value of the Salesforce default 'none'.

Build>Create>Objects>My Object>Page Layouts

This will mean that the standard Salesforce 'none' option on the picklist in the detail page is hidden.