Gst player backend: removed fifo based stream playback fallback

Gst player backend: removed fifo based stream playback fallback

It was used on systems without appsrc element support,
but since the appsrc is part of base elements for a long,
thre is a very little value in fifo based fallback.

Change-Id: I162233cbb55821443c892f00ebd29c0498ae7152
Reviewed-by: Ling Hu <ling.hu@nokia.com>
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>