As of Cuirass commit 9a1452ee021c9f773424961cfeef47ca0b7c5c5a, this option
seems to be unused and kept for back compatibility there.
* gnu/services/cuirass.scm (<cuirass-configuration>): Remove use-substitutes?
field.
(cuirass-shepherd-service): Remove the option.
* doc/guix.texi (Continuous Integration): Remove option documentation.
Change-Id: I933550ce76eecdf918b07891aa8212fd30a7c87e
Signed-off-by: Ludovic Courtès <ludo@gnu.org>