rustc_interface: Remove unused dependency rustc_index

Unused since commit cd7a011
("Don't duplicate the extern providers once for each crate").

Author: joshtriplett

Cargo.lock

@@ -3973,7 +3973,6 @@ dependencies = [
  "rustc_expand",
  "rustc_hir",
  "rustc_incremental",
- "rustc_index",
  "rustc_lint",
  "rustc_metadata",
  "rustc_middle",

compiler/rustc_interface/Cargo.toml

@@ -31,7 +31,6 @@ rustc_codegen_ssa = { path = "../rustc_codegen_ssa" }
 rustc_symbol_mangling = { path = "../rustc_symbol_mangling" }
 rustc_codegen_llvm = { path = "../rustc_codegen_llvm", optional = true }
 rustc_hir = { path = "../rustc_hir" }
-rustc_index = { path = "../rustc_index" }
 rustc_metadata = { path = "../rustc_metadata" }
 rustc_mir = { path = "../rustc_mir" }
 rustc_mir_build = { path = "../rustc_mir_build" }