Remove deprecated support for json job definition

Every users should use a proper yaml job definition. This is taking CPU time
for no reason.

Signed-off-by: Rémi Duraffort <remi.duraffort@linaro.org>
15 jobs for json in 14 minutes and 56 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #60417
amd64
dispatcher-debian-10

00:04:33

passed #60415
amd64
dispatcher-debian-9

00:04:17

passed #60418
amd64
server-debian-10

00:03:53

passed #60416
amd64
server-debian-9

00:04:25

 
  Analyze
passed #60420
amd64
black

00:00:52

passed #60421
amd64
code_quality

00:00:34

passed #60422
amd64
dockerfiles

00:00:33

passed #60423
amd64
job-schema

00:00:36

passed #60424
amd64
pylint

00:03:10

passed #60419
amd64-dind allowed to fail
sast

00:02:54

 
  Build
passed #60427
amd64
amd64/pkg-debian-10

00:02:09

passed #60426
amd64
amd64/pkg-debian-9

00:02:34

passed #60425
amd64
doc

00:01:55

 
  Docker
passed #60428
amd64-dind
amd64/lava-dispatcher

00:02:13

passed #60429
amd64-dind
amd64/lava-server

00:03:42