Removed unnecessary method in NetworkSession

`CompletionStage` that holds current connection can't be failed because
all exceptions are handled when writing to the field. This commit
removes unneeded method that tried to handle all exceptions.

Author: lutovich

driver/src/main/java/org/neo4j/driver/internal/NetworkSession.java

@@ -520,7 +520,7 @@ private CompletionStage<Void> rollbackTransaction()
 
     private CompletionStage<Void> releaseConnection()
     {
-        return existingConnectionOrNull().thenCompose( connection ->
+        return connectionStage.thenCompose( connection ->
         {
             if ( connection != null )
             {
@@ -575,15 +575,10 @@ private CompletionStage<Void> ensureNoOpenTx( String errorMessage )
     private CompletionStage<ExplicitTransaction> existingTransactionOrNull()
     {
         return transactionStage
-                .exceptionally( error -> null ) // handle previous acquisition failures
+                .exceptionally( error -> null ) // handle previous connection acquisition and tx begin failures
                 .thenApply( tx -> tx != null && tx.isOpen() ? tx : null );
     }
 
-    private CompletionStage<Connection> existingConnectionOrNull()
-    {
-        return connectionStage.exceptionally( error -> null ); // handle previous acquisition failures
-    }
-
     private void ensureSessionIsOpen()
     {
         if ( !open.get() )