Remove MaskMut and unused functions from BitBufferMut #7379

Performance Regression: -11.03%

❌ 3 regressed benchmarks
✅ 1119 untouched benchmarks
⏩ 1455 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
❌	Simulation	`true_count_arrow_buffer[128]`	726.4 ns	813.9 ns	-10.75%
❌	Simulation	`true_count_arrow_buffer[1024]`	940.8 ns	1,057.5 ns	-11.03%
❌	Simulation	`true_count_arrow_buffer[2048]`	1 µs	1.1 µs	-10.34%

_{Comparing rk/nomaskmut (b56c88e) with develop (7618c23)}

1455 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩