WASHINGTON - Senate Democrats are quietly trying to kill a 10-year legal probe that implicates several senior Clinton administration appointees for obstruction of justice, the Daily News has learned. The Democrats, saying that the $21 million investigation by Independent Counsel David Barrett should have ended long ago, succeeded in attaching an amendment to a spending bill Tuesday to cut off his funding by June 1. But two sources close to the investigation said that if the legislation becomes law, it will thwart Barrett from making public a final report that names senior officials in the Clinton Justice Department and Internal...