For nearly three decades, Raquel Molina — an immigrant from El Salvador who has a valid Social Security number and permission to work in the United States — swabbed the toilets, wiped down the seats and vacuumed the aisles of airplanes at Boston’s Logan International Airport. But last summer, Ms. Molina, 65, was abruptly fired from her $19.75-per-hour cleaning job, alongside dozens of other immigrants who have long legally worked at Logan.