Remove deprecated INTERVALS_PER_SLOT constant (#4805)

Author: 0xmrree

specs/phase0/fork-choice.md

@@ -105,10 +105,9 @@ handlers must not modify `store`.
 
 ### Constant
 
-| Name                              | Value           |
-| --------------------------------- | --------------- |
-| `INTERVALS_PER_SLOT` *deprecated* | `uint64(3)`     |
-| `BASIS_POINTS`                    | `uint64(10000)` |
+| Name           | Value           |
+| -------------- | --------------- |
+| `BASIS_POINTS` | `uint64(10000)` |
 
 ### Configuration