rustc_data_structures: Drop unused dependency on crossbeam-utils

rustc_data_structures has a dependency on crossbeam-utils but never uses
it. It appears to have originally had this dependency in order to set
the "nightly" feature; however, its other dependencies use a different
version of crossbeam-utils, so this doesn't actually affect anything.
Furthermore, in current crossbeam-utils, the "nightly" feature has
become a no-op.

Author: joshtriplett

Cargo.lock

@@ -3768,7 +3768,6 @@ dependencies = [
  "arrayvec",
  "bitflags",
  "cfg-if 0.1.10",
- "crossbeam-utils 0.8.3",
  "ena",
  "indexmap",
  "jobserver",

compiler/rustc_data_structures/Cargo.toml

@@ -17,7 +17,6 @@ rustc_serialize = { path = "../rustc_serialize" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_graphviz = { path = "../rustc_graphviz" }
 cfg-if = "0.1.2"
-crossbeam-utils = { version = "0.8", features = ["nightly"] }
 stable_deref_trait = "1.0.0"
 rayon = { version = "0.3.1", package = "rustc-rayon" }
 rayon-core = { version = "0.3.1", package = "rustc-rayon-core" }