Is there any working actor with a more powerful, beautiful voice than Broadway’s “Last Great Leading Man,” Brian Stokes Mitchell? The handsome baritone’s performance last night at the Kennedy Center leaves little room for doubt. All gushing aside, Mitchell delivered a wonderful concert, pairing with the National Symphony Orchestra, conducted by another Broadway great, A Chorus Line’s composer Marvin Hamlisch. Though the pair’s banter was occasionally hokey and strained, this hardly detracted from the…