CDRIVER-5703 remove utility headers: bson-atomics.h and bson-cmp.h (#1922)

* remove `bson-atomic.h`
* remove `bson-cmp.h`

Author: kevinAlbs

src/libbson/NEWS

@@ -10,6 +10,8 @@ libbson 2.0.0 (Unreleased)
 
 * `bson_as_json` and `bson_array_as_json` are removed due to producing non-portable Legacy Extended JSON. Prefer Canonical Extended JSON or Relaxed Extended JSON for portability. To continue using Legacy Extended JSON, use `bson_as_legacy_extended_json` and `bson_array_as_legacy_extended_json`.
 * Compiling with `BSON_MEMCHECK` defined now has no effect.
+* Remove deprecated integral comparison interfaces: `bson_in_range_*` and `bson_cmp_*`.
+* Remove deprecated atomic and threading interfaces: `bson_atomic_*` and `bson_thrd_yield`.
 
 libbson 1.30.2
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