Remove HybridBitSet.

It's no longer used.

Author: nnethercote

Cargo.lock

@@ -3820,7 +3820,6 @@ dependencies = [
 name = "rustc_index"
 version = "0.0.0"
 dependencies = [
- "arrayvec",
  "rustc_index_macros",
  "rustc_macros",
  "rustc_serialize",

compiler/rustc_index/Cargo.toml

@@ -5,7 +5,6 @@ edition = "2021"
 
 [dependencies]
 # tidy-alphabetical-start
-arrayvec = { version = "0.7", default-features = false }
 rustc_index_macros = { path = "../rustc_index_macros", default-features = false }
 rustc_macros = { path = "../rustc_macros", optional = true }
 rustc_serialize = { path = "../rustc_serialize", optional = true }