Remove FileHelpers that are not needed

I've removed all methods from FileHelpers that only have one place we can use them.
They are from StorageContainer / TitleContainer mostly.

NormalizeFilePathSeparators remains, in the future something to tidy up the "file:///" code we have in the code base should go in here too, I don't understand it enough yet to do it.

Here is the places all of the functions came from:

FileExists: From MetroHelper.AppDataFileExists
FileOpenRead: TitleContainer.OpenStream
NormalizeFilename: ContentTypeReader.Normalize
SeekStreamToStart: ContentManager.GetContentReaderFromXnb ```#if ANDROID``` section.
DirectoryExists: StorageContainer.DirectoryExists
DirectoryGetFiles: StorageContainer.GetFileNames
DirectoryGetDirectories: StorageContainer.GetDirectoryNames
DirectoryCreate: StorageContainer.CreateDirectory
DirectoryDelete: StorageContainer.DeleteDirectory
GetInstallPath: TitleContainer.Location
OpenStream: ContentManager.OpenStream
StreamClose: Unused
FileDelete: StorageContainer.DeleteFile
FileCreate: StorageContainer.CreateFile
FileOpen: StorageContainer.OpenFile
OpenStreamAsync: TItleContainer.OpenStreamAsync