Remove deprecated "bin/phd ... --gently"
Summary:
Remove deprecated `--gently` parameter for `bin/phd ...` commands. It's been deprecated for more than six years.
Followup to rPb99c240aa3749f0f3042fdf8c2a7553a39dee7c1.
Test Plan: Run e.g. `./bin/phd restart --gently`, now get `Usage Exception: Argument "--gently" is unrecognized.`
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26474