Disabling a cell with a conflicting definition doesn't resolve the conflict

[derikk]

When two cells contain conflicting definitions, I expect that disabling one will resolve the conflict. Instead, both cells get grayed out and the conflict remains.

pluto-bug.mov

[Pangoraw]

Closing this because it seems like a duplicate of #1386. There is an initial attempt to solve this in #1859 but I will require a bit more work to handle all cases (indirectly disabled cells)!