Remove unused usings.

src/Workspaces/Core/Portable/FindSymbols/DeclaredSymbolInfo.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System;
-using System.Collections.Concurrent;
 using System.Collections.Immutable;
 using System.Threading;
 using System.Threading.Tasks;

src/Workspaces/Core/Portable/FindSymbols/SyntaxTree/SyntaxTreeIndex.DeclarationInfo.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System;
-using System.Collections.Concurrent;
 using System.Collections.Immutable;
 using Roslyn.Utilities;