mksrc: Remove references to default_src.config

This file was used in the old times when mksrc.sh used to build
the documentation.

Nowadays this file is useless, so removing it.

Change-Id: I81cb4b8361e2de49a2f8d3914f3cb03cb8ccd7db
Reviewed-by: Simo Fält <simo.falt@digia.com>