A promising pitching prospect is about to join the Mariners straight from Double-A Arkansas. No, sorry, it’s not one of those promising young arms. Kade Anderson and Ryan Sloan, the Mariners’ Nos. 1 and 2 pitching prospects, are both showing dominant potential pitching for the Arkansas Travelers, and both could be in line for a big-league promotion at some point this summer. But not today. The Mariners, instead, announced Saturday that they are promoting right-handed reliever Nick Davila from Double-A to the big-league bullpen. Davila, 27, will make his MLB debut when he appears in a game for the Mariners. The club had an open spot on its 40-man roster. Left-hander Josh Simpson was optioned back to Tacoma, a day after the Mariners promoted him from Triple-A. Davila has a 10-to-1 strikeout-to-walk ratio in his first nine innings for Arkansas this season, with two saves and a hefty 69.6% groundball rate.…