Should there be grease in my steel frame's tubes?
Some mechanic might have put a liberal amount of grease in the BB shell and also in the head-tube prior to assembly. Then copiously greased the seat-post as well. Grease has a tendency to migrate, especially in hot weather and move to the strangest places, the bike may have been stored upside down, head or tail-up.

Think of it as a gift from whoever put it there, the coating also prevented the tubes from rusting on the inside. Wipe off the excess before re-assembly and repack the BB and the headset with fresh grease. In no case would I use solvent to clean the insides of the frame.