plugins/DMM: Use the OSC's IIO context to identify devices

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
plugins: Remove get_context plugin function

This function does not bring any advantage: the OSC could as well use
its own IIO context instead of the plugins'. It also prevented the
plugins to be threaded.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>