Strip out old code

All of these files have been moved to either:

- https://github.com/sigp/lighthouse-beacon
- https://github.com/sigp/lighthouse-validator
- https://github.com/sigp/lighthouse-common

For rationale, see: https://github.com/sigp/lighthouse/issues/197

eth2/utils/slot_clock/src/lib.rs

@@ -1,12 +0,0 @@
-mod system_time_slot_clock;
-mod testing_slot_clock;
-
-pub use crate::system_time_slot_clock::{Error as SystemTimeSlotClockError, SystemTimeSlotClock};
-pub use crate::testing_slot_clock::{Error as TestingSlotClockError, TestingSlotClock};
-pub use types::Slot;
-
-pub trait SlotClock: Send + Sync {
-    type Error;
-
-    fn present_slot(&self) -> Result<Option<Slot>, Self::Error>;
-}