tools: gst-launch: don't try to remove already-removed GSource from main loop
tools: gst-launch: don't try to remove already-removed GSource from main loop

It's considered a programming error in recent GLib versions now.
We may already have removed the source by returning FALSE from
the callback if it was fired. Fixes warning with newer GLibs
when interrupting a pipeline with Control-C.

jpakkane/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,mparis/gstreamer,jpakkane/gstreamer,ensonic/gstreamer,Lachann/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,centricular/gstreamer,surround-io/gstreamer,centricular/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,jpakkane/gstreamer,mparis/gstreamer,ensonic/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,cfoch/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,centricular/gstreamer,krichter722/gstreamer,jpakkane/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,mparis/gstreamer,jpxiong/gstreamer,surround-io/gstreamer,cfoch/gstreamer,ensonic/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,cfoch/gstreamer,cablelabs/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpakkane/gstreamer,centricular/gstreamer,surround-io/gstreamer,krichter722/gstreamer,cfoch/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer