SEATTLE — Chances are, you’ve never heard of Thailand Pierce. And you almost certainly would have missed his recent announcement, such as it was, that he was retiring from professional football. He made that known simply by changing the profile on his Instagram: “Retired Professional Athlete” is the first line of his bio at @tpierce1991. Pierce, 31, has existed on football’s periphery over the past decade, scratching out a living one small paycheck at a time, clawing his way to just about every corner of the country in search of a new team. He went to high school near Dayton, Ohio, and played …