Remove isAsync flag validation from `FileStream` ctors

[adamsitnik]

For the historical reasons described here, .NET was so far throwing if the value of isAsync boolean flag did not match the value returned by SafeFileHandle.IsAsync.

We have agreed to remove these checks:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/FileStream.cs

Lines 91 to 98 in 21d48d1

	if (isAsync && !handle.IsAsync)
	{
	ThrowHelper.ThrowArgumentException_HandleNotAsync(nameof(handle));
	}
	else if (!isAsync && handle.IsAsync)
	{
	ThrowHelper.ThrowArgumentException_HandleNotSync(nameof(handle));
	}

And simply pass the actual value of handle.IsAsync to the layer that chooses the right strategy, just like we do when the flag was not provided by the user:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/FileStream.cs

Line 110 in 21d48d1

_strategy = FileStreamHelpers.ChooseStrategy(this, handle, access, bufferSize, handle.IsAsync);

So the following lines need to be updated:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/FileStream.cs

Line 117 in 21d48d1

_strategy = FileStreamHelpers.ChooseStrategy(this, handle, access, bufferSize, isAsync);

runtime/src/libraries/System.Private.CoreLib/src/System/IO/FileStream.cs

Line 51 in 21d48d1

_strategy = FileStreamHelpers.ChooseStrategy(this, safeHandle, access, bufferSize, isAsync);

And all unused code (like the throw helpers) needs to be removed as well. The tests also need to be updated accordingly.

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