Simplify IndexReader constructor

[paulirwin]

The IndexReader constructor is:

lucenenet/src/Lucene.Net/Index/IndexReader.cs

Lines 84 to 90 in b1476ae

	private protected IndexReader() // LUCENENET: Changed from internal to private protected
	{
	if (!(this is CompositeReader || this is AtomicReader))
	{
	throw Error.Create("IndexReader should never be directly extended, subclass AtomicReader or CompositeReader instead.");
	}
	}

We can remove the check and error message, since it is declared private protected. But we will need a LUCENENET specific comment to indicate we removed it in the .NET version and enforced it at the compiler level. There may also be tests to confirm this exception is thrown.

This can be another issue that is outside of the scope of this PR.

Originally posted by @NightOwl888 in #914 (comment)