rustc_symbol_mangling: Remove unused dependency rustc_ast

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

Author: joshtriplett

Cargo.lock

@@ -4373,7 +4373,6 @@ version = "0.0.0"
 dependencies = [
  "punycode",
  "rustc-demangle",
- "rustc_ast",
  "rustc_data_structures",
  "rustc_hir",
  "rustc_middle",

compiler/rustc_symbol_mangling/Cargo.toml

@@ -12,7 +12,6 @@ tracing = "0.1"
 punycode = "0.4.0"
 rustc-demangle = "0.1.18"
 
-rustc_ast = { path = "../rustc_ast" }
 rustc_span = { path = "../rustc_span" }
 rustc_middle = { path = "../rustc_middle" }
 rustc_hir = { path = "../rustc_hir" }