ByteHelpers.ToHex: Use .NET implementation

[kiminuo]

Simple https://benchmarkdotnet.org benchmark confirms that we no longer need the custom code:

[MemoryDiagnoser]
public class Benchmarks
{
	private byte[] _bytes;

	[Params(4, 16, 128)]
	public int Length { get; set; }

	[GlobalSetup]
	public void Setup() => _bytes = RandomNumberGenerator.GetBytes(Length);


	[Benchmark]
	public string Master()
	{
		return WalletWasabi.Helpers.ByteHelpers.ToHex(_bytes);
	}

	[Benchmark]
	public string Pr()
	{
		return Convert.ToHexString(_bytes);
	}
}

Results:

Method	Length	Mean	Error	StdDev	Median	Gen 0	Gen 1	Allocated
Master	4	11.18 ns	0.314 ns	0.911 ns	10.91 ns	0.0063	-	40 B
Pr	4	12.01 ns	0.271 ns	0.628 ns	11.98 ns	0.0064	-	40 B
Master	16	18.84 ns	0.398 ns	0.332 ns	18.91 ns	0.0140	-	88 B
Pr	16	16.43 ns	0.361 ns	0.632 ns	16.26 ns	0.0140	-	88 B
Master	128	101.23 ns	1.651 ns	2.027 ns	100.90 ns	0.0854	0.0001	536 B
Pr	128	55.80 ns	1.124 ns	2.056 ns	55.38 ns	0.0854	0.0002	536 B