Simplify IndexReader constructor, #919

[paulirwin]

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Simplify IndexReader constructor.

Fixes #919

Description

This removes the type check in the IndexReader constructor that ensures that only CompositeReader and AtomicReader are allowed subtypes. This is not needed since in C# we can use the private protected modifier to ensure that no outside classes can subtype this. This removes the need for the runtime check in the constructor, which will mildly improve performance. A comment was left explaining the divergence with the original commented-out code.