Move in common a property with an undue discrepancy.

This is not fixing internal troubles since this property has no usages.

Author: fredericDelaporte

src/NHibernate/Impl/AbstractSessionImpl.cs

@@ -413,7 +413,9 @@ protected bool IsAlreadyDisposed
 
 		public abstract void Flush();
 
-		public abstract bool TransactionInProgress { get; }
+		// 6.0 TODO: remove virtual.
+		/// <inheritdoc />
+		public virtual bool TransactionInProgress => ConnectionManager.IsInActiveTransaction;
 
 		#endregion
 

src/NHibernate/Impl/SessionImpl.cs

@@ -1326,11 +1326,6 @@ public override void Flush()
 			}
 		}
 
-		public override bool TransactionInProgress
-		{
-			get { return ConnectionManager.IsInActiveTransaction; }
-		}
-
 		public bool IsDirty()
 		{
 			using (BeginProcess())

src/NHibernate/Impl/StatelessSessionImpl.cs

@@ -369,11 +369,6 @@ public void ManagedFlush()
 			}
 		}
 
-		public override bool TransactionInProgress
-		{
-			get { return Transaction.IsActive; }
-		}
-
 		#region IStatelessSession Members
 
 		public override CacheMode CacheMode