NEW YORK — Thursday was Roberto Clemente Day across Major League Baseball. With the Pittsburgh Pirates — Clemente’s old team — in town for a nationally televised game, the Mets and Pirates were the only teams in the league to have every player wear Clemente’s iconic No. 21 on their backs. Every MLB team had a 21 patch on their jerseys on Thursday and Puerto Rican players as well as past nominees for the Roberto Clemente Award were also permitted to wear 21 instead of their regular number. “It’s still sad to think about how he passed,” Mets manager Buck Showalter said before the game. “But, it’…