Remove extern crate (#5922)

* Remove extern crate

common/slot_clock/src/lib.rs

@@ -1,6 +1,3 @@
-#[macro_use]
-extern crate lazy_static;
-
 mod manual_slot_clock;
 mod metrics;
 mod system_time_slot_clock;

common/slot_clock/src/metrics.rs

@@ -1,4 +1,5 @@
 use crate::SlotClock;
+use lazy_static::lazy_static;
 pub use lighthouse_metrics::*;
 use types::{EthSpec, Slot};