WASHINGTON — The share of federal drug offenders subject to mandatory minimum sentences that critics call unduly harsh has plunged in the last year, according to figures Attorney General Eric Holder planned to release today in arguing for the success of his criminal justice policies. Preliminary data obtained by The Associated Press show a 6 percent drop in the number of federal drug trafficking prosecutions in a one-year period ending in September. At the same time, the percentage of drug cases in which …