[C#] cleanup securememory

[nikoo28]

there are a couple of pending cleanup tasks in the Windows implementation for C#

• found some unused imports
• the visibility of some methods can be made private
• remove BOM/CLRF in /csharp/SecureMemory/PlatformNative/LLP64/Windows/WindowsInterop.cs
• remove BOm/CLRF in /csharp/SecureMemory/SecureMemory/ProtectedMemoryImpl/Windows/WindowsProtectedMemoryAllocatorLLP64.cs

[jgowdy]

This is resolved in #300

[chief-micco]

This may have been handled in the recent update from #1416