chore: delete owlbot.py

[parthea]

Now that googleapis/synthtool#1244 is merged, owlbot.py is no longer required in this repo. We can add owlbot.py back in the future if repository specific customizations are needed.

[parthea]

Wait until the following lines are added to synthtool common code

for noxfile in REPO_ROOT.glob("samples/**/noxfile.py"):
    s.shell.run(["nox", "-s", "blacken"], cwd=noxfile.parent, hide_output=False)

[parthea]

Wait for googleapis/synthtool#1276

[parthea]

googleapis/synthtool#1276 was merged.