`RequestRetryType.RetryOnStreamLimitReached` is never used

[MihaZupan]

We have this catch block looking for RequestRetryType.RetryOnStreamLimitReached

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.cs

Lines 537 to 545 in 3f7c3d2

	catch (HttpRequestException e) when (e.AllowRetry == RequestRetryType.RetryOnStreamLimitReached)
	{
	if (NetEventSource.Log.IsEnabled())
	{
	Trace($"Retrying request on another HTTP/2 connection after active streams limit is reached on existing one: {e}");
	}
	// Eat exception and try again.
	}

but since .NET 6 (#53851), the flag isn't set anywhere, so this is all dead code.

Noticed while looking at dotnet/arcade#16305

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