rustc_metadata: Remove unused dependency stable_deref_trait

Unused since commit 8331dbe
("Add an Mmap wrapper to rustc_data_structures").

Author: joshtriplett

Cargo.lock

@@ -4083,7 +4083,6 @@ dependencies = [
  "rustc_target",
  "smallvec",
  "snap",
- "stable_deref_trait",
  "tracing",
  "winapi 0.3.9",
 ]

compiler/rustc_metadata/Cargo.toml

@@ -23,7 +23,6 @@ rustc_target = { path = "../rustc_target" }
 rustc_index = { path = "../rustc_index" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_serialize = { path = "../rustc_serialize" }
-stable_deref_trait = "1.0.0"
 rustc_ast = { path = "../rustc_ast" }
 rustc_expand = { path = "../rustc_expand" }
 rustc_span = { path = "../rustc_span" }