[content] fixed warnings

[content] fixed warnings

[content] removed unused variables from WorkThread (content_instance.cc)
[content] removed unused variable from ContentInstance::ContentManagerCreatPlaylist
[content] added checking of returned values from media_folder_* functions in FolderToJson (content_manager.cc)
[content] fixed unsigned with signed int comparison warning in ContentManager::playlistRemovebatch
[content] fixed unsigned with signed int comparisons warning in ContentManager::playlistSetOrder
[content] added checking of returned value from media_playlist_get_media_count_from_db in ContentManager::playlistSetOrder
[content] removed unused function int get_utc_offset() (content_manager.cc)

Change-Id: Ia274e4542930099a2aaea77c0f0f9d14bdce6053
Signed-off-by: Maciej Kulawik <m.kulawik@samsung.com>
(cherry picked from commit 62bd07282eabe60f9347f00b69610517081f6e53)