When the Census Bureau hired upward of 700,000 Americans over the last two years — most in the last six months — it landed more experienced workers with more sophisticated skills than any time in recent memory. This was the unintended upside of the nastiest recession of the last 70 years. Now, its decennial work largely done, the Census Bureau is shedding hundreds of thousands of workers — about 225,000 in just the last few weeks, enough to account for a jot or two in the unemployment rate, say federal economists. Most of those remaining will be gone by August; a few will last into September.