[maguro] Move GST_OMX_CONFIG_DIR to a profile.d script and remove GST_OMX_EXTENSION env var

GST_OMX_EXTENSION is not needed with changes in the gst-omx fork.
Also, setting GST_OMX_CONFIG_DIR causes issues when running the browser,
specifically it appears that the plugin gets removed/blacklisted from the
GStreamer registry if the plugin loading fails, which happens in the browser,
resulting in videos not running in the Gallery app afterwards.

Change-Id: Id16910f501f0ed1ea150cc5cbf1bd72ce347676a