Remove the single_threaded/multi_threaded feature flags (#1390)

Always use parking_lot for mutexes, i.e. always be multi-threaded.

Closes #1379

Cargo.lock

@@ -203,12 +203,6 @@ dependencies = [
  "system-deps",
 ]
 
-[[package]]
-name = "atomic_refcell"
-version = "0.1.8"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "73b5e5f48b927f04e952dedc932f31995a65a0bf65ec971c74436e51bf6e970d"
-
 [[package]]
 name = "atty"
 version = "0.2.14"
@@ -1206,7 +1200,6 @@ version = "0.17.0"
 dependencies = [
  "ab_glyph",
  "ahash 0.7.6",
- "atomic_refcell",
  "bytemuck",
  "cint",
  "criterion",