The DTS variable has been removed in 402138d12dca ("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>
c4f6850efc