mv_machine_learning: change feature vector manager for face recognition

mv_machine_learning: change feature vector manager for face recognition

[Issue type] : code enhancement

Change feature vector manager including some code cleanups for face recognition
task group.

With this patch, the header data of the feature vector is now stored at the
beginning of the feature vector file, while the feature vector data is stored
at the end of the file. This change makes it easier to manage feature vectors
without having to worry about moving the file pointer.
In the previous version, the header data was stored at the end of the file,
and the feature vector data was stored at the beginning of the file.

Additionally, this patch skips an unnecessary process in the deleteLabel
function and cleans up the existing code.

Change-Id: I9ef2c0f5603788ea1907002bacf25d9f4fba68ed
Signed-off-by: Inki Dae <inki.dae@samsung.com>