op-mode: T6537: remove unused cmd imported from vyos.utils.process

Commit dc60fe9 ("op-mode: T6537: include hostname in the reboot/shutdown
warning message") added a more local import of vyos.utils.process.cmd() that
made the fglobal import obsolete and trigger a linter warning.

$ make unused-imports

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Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

(cherry picked from commit 6b2e45c)

src/op_mode/powerctrl.py

@@ -24,7 +24,6 @@
 
 from vyos.utils.io import ask_yes_no
 from vyos.utils.process import call
-from vyos.utils.process import cmd
 from vyos.utils.process import run
 from vyos.utils.process import STDOUT