configure: drop never-used RTAI3_MOD variable
configure: drop never-used RTAI3_MOD variable

this may once have been used to dynamically list the rtai modules to be
loaded, but this list has been static for a long time.

Signed-off-by: Jeff Epler <623a871f32baaca066adfe0cac08d7aa7d34027d@unpythonic.net>

ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,bmwiedemann/linuxcnc-mirror,ikcalB/linuxcnc-mirror,ikcalB/linuxcnc-mirror