Remove redundant column names collection from DataFrameColumnCollection

# Conflicts:
#	src/Microsoft.Data.Analysis/DataFrameColumnCollection.cs

Author: asmirnov82

src/Microsoft.Data.Analysis/DataFrameColumnCollection.cs

@@ -14,9 +14,6 @@ namespace Microsoft.Data.Analysis
     public class DataFrameColumnCollection : Collection<DataFrameColumn>
     {
         private readonly Action ColumnsChanged;
-
-        private readonly List<string> _columnNames = new List<string>();
-
         private readonly Dictionary<string, int> _columnNameToIndexDictionary = new Dictionary<string, int>(StringComparer.Ordinal);
 
         internal long RowCount { get; set; }
@@ -58,7 +55,6 @@ internal void UpdateColumnNameMetadata(DataFrameColumn column, string newName)
         {
             string currentName = column.Name;
             int currentIndex = _columnNameToIndexDictionary[currentName];
-            _columnNames[currentIndex] = newName;
             _columnNameToIndexDictionary.Remove(currentName);
             _columnNameToIndexDictionary.Add(newName, currentIndex);
             ColumnsChanged?.Invoke();
@@ -93,11 +89,10 @@ protected override void InsertItem(int columnIndex, DataFrameColumn column)
 
             column.AddOwner(this);
 
-            _columnNames.Insert(columnIndex, column.Name);
             _columnNameToIndexDictionary[column.Name] = columnIndex;
             for (int i = columnIndex + 1; i < Count; i++)
             {
-                _columnNameToIndexDictionary[_columnNames[i]]++;
+                _columnNameToIndexDictionary[this[i].Name]++;
             }
             base.InsertItem(columnIndex, column);
             ColumnsChanged?.Invoke();
@@ -115,9 +110,7 @@ protected override void SetItem(int columnIndex, DataFrameColumn column)
             {
                 throw new ArgumentException(string.Format(Strings.DuplicateColumnName, column.Name), nameof(column));
             }
-
-            _columnNameToIndexDictionary.Remove(_columnNames[columnIndex]);
-            _columnNames[columnIndex] = column.Name;
+            _columnNameToIndexDictionary.Remove(this[columnIndex].Name);
             _columnNameToIndexDictionary[column.Name] = columnIndex;
 
             this[columnIndex].RemoveOwner(this);
@@ -128,12 +121,11 @@ protected override void SetItem(int columnIndex, DataFrameColumn column)
 
         protected override void RemoveItem(int columnIndex)
         {
-            _columnNameToIndexDictionary.Remove(_columnNames[columnIndex]);
+            _columnNameToIndexDictionary.Remove(this[columnIndex].Name);
             for (int i = columnIndex + 1; i < Count; i++)
             {
-                _columnNameToIndexDictionary[_columnNames[i]]--;
+                _columnNameToIndexDictionary[this[i].Name]--;
             }
-            _columnNames.RemoveAt(columnIndex);
 
             this[columnIndex].RemoveOwner(this);
             base.RemoveItem(columnIndex);
@@ -171,7 +163,6 @@ protected override void ClearItems()
         {
             base.ClearItems();
             ColumnsChanged?.Invoke();
-            _columnNames.Clear();
             _columnNameToIndexDictionary.Clear();
 
             //reset RowCount as DataFrame is now empty