Remove an unneeded dependency.

Author: nnethercote

Cargo.lock

@@ -4054,7 +4054,6 @@ dependencies = [
  "rustc_hir_analysis",
  "rustc_hir_typeck",
  "rustc_incremental",
- "rustc_index",
  "rustc_lint",
  "rustc_macros",
  "rustc_metadata",

compiler/rustc_interface/Cargo.toml

@@ -26,7 +26,6 @@ rustc_middle = { path = "../rustc_middle" }
 rustc_ast_lowering = { path = "../rustc_ast_lowering" }
 rustc_ast_passes = { path = "../rustc_ast_passes" }
 rustc_incremental = { path = "../rustc_incremental" }
-rustc_index = { path = "../rustc_index" }
 rustc_traits = { path = "../rustc_traits" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_codegen_ssa = { path = "../rustc_codegen_ssa" }