Lionel Cote, the chief of police in Lehighton since Aug. 3, 1981, is resigning to become head of a police department in the Melbourne Beach area, on the east cost of Florida. In a letter of resignation submitted this morning to Mayor Clinton Williams, Cote said he has received several offers "that I could not refuse, and so I have decided to leave at this time to further my professional growth." He will work his last day in Lehighton on Oct. 18 and assume his new duties on Oct. 21.