rustc_lint_defs: Remove apparently unused dependency on tracing (as "log")

Author: joshtriplett

Cargo.lock

@@ -4040,7 +4040,6 @@ dependencies = [
  "rustc_serialize",
  "rustc_span",
  "rustc_target",
- "tracing",
 ]
 
 [[package]]

compiler/rustc_lint_defs/Cargo.toml

@@ -5,7 +5,6 @@ version = "0.0.0"
 edition = "2018"
 
 [dependencies]
-log = { package = "tracing", version = "0.1" }
 rustc_ast = { path = "../rustc_ast" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_span = { path = "../rustc_span" }