Unused Indexes - Consider Primary Key Constraint Supporting Indexes?
Primary Key and Unique constraints are enforced by a supporting index for performance and to enforce uniqueness. Just because the index is not used for searching does not mean it isn't being used to guarantee uniqueness of the key values.

Whether or not a table has a primary key constraint or any unique constraints is not a performance decision - it is a data integrity decision. The primary key gets a primary key constraint and the candidate keys get unique constraints.

Whether or not the primary key index is clustered or nonclustered, on the other hand, is a performance question. Doing a Binoogle search on that will find many hits, including https://stackoverflow.com/questions/2206541/index-view-how-to-choose-the-clustered-index.