Remove the unnecessary Set and use enum directly.

### Description

Currently, Spillers declare two set.
```
public static final Set<Spiller.Phase> PHASE_SET_SHRINK_ONLY =
    Collections.singleton(Spiller.Phase.SHRINK);

public static final Set<Spiller.Phase> PHASE_SET_SPILL_ONLY =
    Collections.singleton(Spiller.Phase.SPILL);
```
After my investigation, we can avoid using Set here, Enum is better way.

### Gluten version

None