ST. LOUIS — A former FBI agent who investigated former Missouri Gov. Eric Greitens in 2018 pleaded guilty Wednesday to misdemeanor evidence tampering, avoiding a perjury trial that would have started with jury selection Thursday. William Don Tisaby, 69, appeared before St. Louis Circuit Judge Bryan Hettenbach at 1:30 p.m. A special prosecutor filed a superseding charge of misdemeanor evidence tampering shortly before the plea, replacing a variety of other charges, including perjury. Tisaby received a suspended sentence of one year of probation and will be required to pay court costs. Johnson C…