Years ago I asked my dad about it and he told me that there were 4 reasons they were removed.
A. They tended to cause drag when damaged.
B. They could jam control surfaces when damaged and began flapping in the wind.
C. As 95% of the times the weather was bad enough to need them, you weren't going to be flying anyway (flying for long periods in clouds with a 1000 or so other large aircraft in relativly close formation gets interesting I guess)
D. The weight of the deicer systems (Boots, pumps, lines etc) could be better used in fuel and ammo.
If you think about it, if you get rid of the bombs, an iced up Liberator or Fortress is far more likely to survive an encounter with ice than any figher (I don't know of any civilian P-51s that are "Approved for Operation in Known Icing Conditions" by the FAA)