Here are the results of my investigation, based on the details you asked for.
1.) By using Quick Find, I searched for the term CrystalDecisions.Enterprise.Framework, with zero results for the entire solution. I then went form by form, dialogbox by dialog box to verify that nothing is using/referencing CrystalDecisions.Enterprise.Framework.
3.) I do not have a web.config, but a quick look into app.config turned up zero results also.
Additional information ...
4.) While digging around, I discovered that a coworker had included a form which utilized Crystal Reports Viewer, but had since removed it from Source Control. The control was still showing up in Source Control as a "Ghost" component, so I removed it (with crossed-fingers - and hoping that it would resolve my issues). But after Cleaning, Rebuilding ... I still have the same problem. Those four warning, which can be cleared by removing unused references - and then 30 seconds later, they come back.
With the weekend coming up on us, I am making my preparations to leave work, so it won't be until Monday morning before I can get back to you. Over the weekend I will do some more poking and prodding around to see what I can find. I do appreciate your assistance very much.
Thank you and have a great weekend.
Phil Delay