chore(*): cleanup imports (#3511)
A not-very-interesting cleanup of imports.
I deleted
```
instance orbit_fintype (b : β) [fintype α] [decidable_eq β] :
fintype (orbit α b) := set.fintype_range _
```
which wasn't being used, for the sake of not having to import everything about finiteness into `algebra.group_action`.
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
