remove unused backtrace refs

Cargo.lock

@@ -5581,7 +5581,3 @@ checksum = "39f0c922f1a334134dc2f7a8b67dc5d25f0735263feec974345ff706bcf20b0d"
 dependencies = [
  "linked-hash-map",
 ]
-
-[[patch.unused]]
-name = "backtrace"
-version = "0.3.55"

Cargo.toml

@@ -106,11 +106,5 @@ rustc-std-workspace-core = { path = 'library/rustc-std-workspace-core' }
 rustc-std-workspace-alloc = { path = 'library/rustc-std-workspace-alloc' }
 rustc-std-workspace-std = { path = 'library/rustc-std-workspace-std' }
 
-# This crate's integration with libstd is a bit wonky, so we use a submodule
-# instead of a crates.io dependency. Make sure everything else in the repo is
-# also using the submodule, however, so we can avoid duplicate copies of the
-# source code for this crate.
-backtrace = { path = "library/backtrace" }
-
 [patch."https://github.com/rust-lang/rust-clippy"]
 clippy_lints = { path = "src/tools/clippy/clippy_lints" }

src/tools/tidy/src/deps.rs

@@ -67,7 +67,6 @@ const PERMITTED_DEPENDENCIES: &[&str] = &[
     "arrayvec",
     "atty",
     "autocfg",
-    "backtrace",
     "bitflags",
     "block-buffer",
     "block-padding",