rustc_infer: Remove unused dependency rustc_ast

Unused since commit 50e1ae1
("Use ty::{IntTy,UintTy,FloatTy} in rustc").

Author: joshtriplett

Cargo.lock

@@ -3940,7 +3940,6 @@ dependencies = [
 name = "rustc_infer"
 version = "0.0.0"
 dependencies = [
- "rustc_ast",
  "rustc_data_structures",
  "rustc_errors",
  "rustc_hir",

compiler/rustc_infer/Cargo.toml

@@ -20,4 +20,3 @@ rustc_serialize = { path = "../rustc_serialize" }
 rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }
 smallvec = { version = "1.6.1", features = ["union", "may_dangle"] }
-rustc_ast = { path = "../rustc_ast" }