Is it permissible to gradually reduce ties with friends who are not from one's tribe, even though they are among the best of people in character, and would that be considered a sin, arrogance, or an act of the جاهلية (pre-Islamic era of ignorance)?

Islam organizes a Muslim's relationship with his brethren based on piety, not on color or lineage. Affiliation with a tribe or group is for acquaintance and cooperation, not for boasting. What is forbidden is ignorant partisanship. Those who are partisan must be advised and the truth clarified to them. One must be patient with good company regardless of their lineage, and if one cannot endure harm, there is no sin in leaving their company without boycotting or harming them. However, patience is preferable for the sake of upholding the truth.