Revert "Lucene.Net.Support.WeakDictionary: Changed backing collection from J2N.Collections.Generic.Dictionary<TKey, TValue> to ConcurrentDictionary<TKey, TValue> to allow reap to function while doing a forward iteration through the collection."
Revert "Lucene.Net.Support.WeakDictionary: Changed backing collection from J2N.Collections.Generic.Dictionary<TKey, TValue> to ConcurrentDictionary<TKey, TValue> to allow reap to function while doing a forward iteration through the collection."

This reverts commit ddfb7b2108f0d72fd03c563c4bfc2f3479cdf91f.

This was causing occasional test failures of Lucene.Net.Support.TestWeakDictionary::C_TestMemLeakage().

apache/lucenenet,NightOwl888/lucenenet,jeme/lucenenet,NightOwl888/lucenenet,apache/lucenenet,apache/lucenenet,jeme/lucenenet,NightOwl888/lucenenet,NightOwl888/lucenenet,apache/lucenenet,jeme/lucenenet,jeme/lucenenet