Deep inside Alex Mack’s thank-you note for 13 seasons in the NFL was a compelling line, one that aptly describes life as an offensive lineman, whether they’re highly accomplished like him or not. “I think I’ll miss the O-line rooms the most,” Mack wrote in a retirement announcement Friday on Twitter and Instagram. “Always a special collection of men willing to put in the work together, in near anonymity, for the good of the team.” Drafted out of Cal in 2009 as a Cleveland Browns first-round pick, Mack returned to the Bay Area for a victory lap, of sorts, to finish his career as the 49ers’ cent…