rustc_infer: Remove unused dependency rustc_graphviz

Unused since commit 7d73e4c
("Remove ReScope").

Author: joshtriplett

Cargo.lock

@@ -3943,7 +3943,6 @@ dependencies = [
  "rustc_ast",
  "rustc_data_structures",
  "rustc_errors",
- "rustc_graphviz",
  "rustc_hir",
  "rustc_index",
  "rustc_macros",

compiler/rustc_infer/Cargo.toml

@@ -8,7 +8,6 @@ edition = "2018"
 doctest = false
 
 [dependencies]
-rustc_graphviz = { path = "../rustc_graphviz" }
 tracing = "0.1"
 rustc_middle = { path = "../rustc_middle" }
 rustc_data_structures = { path = "../rustc_data_structures" }