The DTS variable has been removed in 402138d1 ("ramips: Derive
DTS name from device name in Makefile"), but the DEVICE_VARS entry
has been overlooked.

Remove it now since we are not using this variable.

This must _not_ be backported to 19.07, where the variable is still
in use.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>