ARLINGTON, Texas — The decision to designate veteran outfielder Randal Grichuk for assignment Wednesday came just as the 34-year-old was starting to heat up, batting 6-for-16 after starting the year 0-for-15. The Yankees had signed the right-handed hitter to a minor league deal in camp before he made the team to help them against lefties. “Difficult decision, just because I feel like Randal’s come in here and fit in really well,” Aaron Boone said before the 3-0 loss to the Rangers . “I feel like, despite numbers-wise a slow start, he’s shown us the at-bats we’ve wanted to see from him when he’s got his opportunities against left-handed pitching. Yankees left fielder Randal Grichuk (34) hits a double to drive in a run against the Boston Red Sox.…