Remove benchmark_bson.py and clarify Rust testing in perf_test.py

The benchmark_bson.py script was redundant with the existing Rust comparison
tests in perf_test.py. Updated README to clarify that Rust vs C benchmarking
happens in perf_test.py with dedicated test classes:

- TestRustSimpleIntEncoding[C|Rust]
- TestRustSimpleIntDecoding[C|Rust]
- TestRustMixedTypesEncoding[C|Rust]
- TestRustNestedEncoding[C|Rust]
- TestRustListEncoding[C|Rust]

This provides a cleaner, more maintainable approach with all performance
tests in one place.

Author: aclark4life

README.md

@@ -559,28 +559,27 @@ PYMONGO_USE_RUST=1 pytest test/test_bson.py -v
 
 **Benchmark BSON performance:**
 
-A focused BSON benchmark script is available at `test/performance/benchmark_bson.py`:
+The performance test suite at `test/performance/perf_test.py` includes Rust vs C comparison benchmarks:
 
 ```bash
-# Compare C vs Rust extensions (default)
-python test/performance/benchmark_bson.py
+# Run all performance tests (includes BSON micro-benchmarks with C vs Rust comparison)
+FASTBENCH=1 python test/performance/perf_test.py -v
 
-# Quick test with fewer iterations
-python test/performance/benchmark_bson.py --quick
+# Run only Rust comparison tests
+FASTBENCH=1 python test/performance/perf_test.py -v -k Rust
 
-# Verbose output
-python test/performance/benchmark_bson.py -v
-
-# Test only C extension
-python test/performance/benchmark_bson.py --c-only
-
-# Test only Rust extension
-python test/performance/benchmark_bson.py --rust-only
+# Run specific Rust comparison tests
+FASTBENCH=1 python test/performance/perf_test.py -v TestRustSimpleIntEncodingC TestRustSimpleIntEncodingRust
 ```
 
-**Difference from perf_test.py:**
-- `benchmark_bson.py`: Focused BSON encoding/decoding benchmarks only (no database, just serialization)
-- `perf_test.py`: Full MongoDB driver performance suite (includes network, database operations, BSON, etc.)
+**Rust comparison tests in perf_test.py:**
+- `TestRustSimpleIntEncoding[C|Rust]` - Simple integer document encoding
+- `TestRustSimpleIntDecoding[C|Rust]` - Simple integer document decoding
+- `TestRustMixedTypesEncoding[C|Rust]` - Mixed types (string, int, float, bool, null)
+- `TestRustNestedEncoding[C|Rust]` - Nested document encoding
+- `TestRustListEncoding[C|Rust]` - List encoding
+
+Each test has both a C and Rust variant for direct comparison. The test suite also includes standard BSON micro-benchmarks (flat, deep, full) and full MongoDB driver benchmarks.
 
 ### Technical Details