There actually was a reason given for it's removal. I am taking this from the 'Freljord AMA' that happened yesterday.
[ Why removal Warmogs armor from ARAM? Warmogs countered poke teams and gave melee a chance. Why would you remove it?]
"Warmog's didn't counter poke teams; poke teams dominated early game, and used that advantage to snowball into late game. What Warmog's did was counter ranged teams, and allows melee champions toeventually become useful, but among good players, almost always win. We added the Guardian's Horn to fulfill the niche that a health/regen item should have filled early, but still remain useful mfrom mid-to-late game." - Nome