Rip out all code that directly uses gstreamer

We can now rip CDs without gstreamer.

This is not the most clean attempt, but I have tried to remove most of the
code that depends on gstreamer. I hope there is not a lot of code left that
depends on code that I have removed - I can at least rip a CD fully.

morituri/common/task.py

@@ -6,7 +6,7 @@ import signal
 import subprocess
 
 from morituri.extern import asyncsub
-from morituri.extern.task import task, gstreamer
+from morituri.extern.task import task
 
 import logging
 logger = logging.getLogger(__name__)
@@ -24,10 +24,6 @@ class LoggableMultiSeparateTask(task.MultiSeparateTask):
     pass
 
 
-class GstPipelineTask(gstreamer.GstPipelineTask):
-    pass
-
-
 class PopenTask(task.Task):
     """
     I am a task that runs a command using Popen.