package/ubus: remove obsolete systemd option

package/ubus: remove obsolete systemd option

ENABLE_SYSTEMD option has been removed from ubus by upstream commit:

  96ab0b3032f5 ubusd: remove systemd socket activation support

From a Buildroot perspective, this means that this systemd socket
activation feature no longer exists since Buildroot commit
130be80d34905d3e79ff6b8f8fb31b3c3765eb96 ("ubus: bump version"), as we
bumped ubus from 259450f414d8c9ee41896e8e6d6bc57ec00e2b63 to
34c6e818e431cc53478a0f7c7c1eca07d194d692 in this commit, and the
96ab0b3032f5 ("ubusd: remove systemd socket activation support")
commit is in this range. It was therefore dropped upstream in 2016,
and in Buildroot in 2017.

Signed-off-by: Jan Havran <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>