Display the Name of the Marketing Campaign only when Selecting a Lead Source in the Opportunity Record

When Auto-Generated Numbers feature is enabled for Campaign records, the values on the Lead Source field displays both the prefixed number and the name of the Marketing Campaign.

This is by design and the Enhancement request # 109145 is filed for it:
Campaigns > Ability to display the campaign name without the campaign ID in the lead source list.

The alternate solution is presented below.

1. As Administrator navigate to Setup > Company > Auto-Generated Numbers.
2. In the CRM subtab > Deselect the Enable box for Campaign. 
3. Click Save.
4. Navigate to Lists > Marketing > Marketing Campaigns.
5. Edit the Campaigns and remove the numbers in the ID field.
6. Click Save.
7. Edit an Opportunity record > Sales subtab.

The values in the Lead Source field now displays the name of the Marketing Campaign only.