The acting head of the U.S. immigration court system and three other top officials were fired on Monday soon after President Trump took office, according to three people familiar with the matter, in a purge of the top echelon of a critical part of the government’s immigration system. The abrupt removals signaled that the Trump administration wants to remake the immigration court system, which is housed under the Justice Department, as part of a broader immigration crackdown that Mr. Trump began within minutes of being sworn in for his second term. Immigration judges oversee an essential part of the system:...