[mediacontroller][5.5] fix method to update playback item

[mediacontroller][5.5] fix method to update playback item

[ACR]
http://suprem.sec.samsung.net/jira/browse/TWDAPI-240

[description]
removed checking if playlist exists on device, because
some 3rd party application can get the playlist
from the 3rd party server(such as cloud) directly and we
don't have any information about its name and index

[verification]
MediaControllerServer_updatePlaybackItem_invalid.html - only
this testcase fails (from tct-mediacontroller-tizen-tests).
It needs to be changed/removed

Change-Id: I7809b662a2eb9b99af3690892133e85ed9b41865
Signed-off-by: Dawid Juszczak <d.juszczak@samsung.com>