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

Commit dc60fe99350 ("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

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

(cherry picked from commit 6b2e45c073eeef62bbb5905e1bff98e20199b6b0)