Remove extern crate tracing.

`use` is a nicer way of doing things.

compiler/rustc_data_structures/src/profiling.rs

@@ -99,6 +99,7 @@ pub use measureme::EventId;
 use measureme::{EventIdBuilder, Profiler, SerializableString, StringId};
 use parking_lot::RwLock;
 use smallvec::SmallVec;
+use tracing::warn;
 
 bitflags::bitflags! {
     #[derive(Clone, Copy)]