buildkite: drop explicit "depends_on": None step entries

These were needed when the we still had a `wait` step (to overwrite the
`wait` step and instead start immediately), however since #4803 we do
not have a `wait` step anymore (instead everything that used to be after
the wait step now explicitly depends on the compilation steps). Thus,
having the `depends_on: null` entries for kani and style steps is no
longer needed (it also does not harm, but it looks ugly in buildkite's
canvas view).

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

.buildkite/pipeline_pr.py

@@ -28,7 +28,6 @@
     {
         "command": "./tools/devtool -y checkstyle",
         "label": "🪶 Style",
-        "depends_on": None,
     },
     depends_on_build=False,
 )
@@ -66,7 +65,6 @@
     # modify Kani steps' label
     for step in kani_grp["steps"]:
         step["label"] = "🔍 Kani"
-    kani_grp["depends_on"] = None
 
 if run_all_tests(changed_files):
     pipeline.build_group(