sysutils/rundeck: Fix rc script startup

The fix is to remove ${rc_arg} from the su invocation in rundeck_start().
The JAR is invoked directly via daemon(8) and does not need nor support
rc(8)-style subcommands.

PR: 294978
Reported by: Steffen <steffen@fritz.wtf>
Approved by: Xavier Humbert <xavier@groumpf.org> (maintainer)
MFH: 2026Q2

(cherry picked from commit 36b5f8761acf2eaec5ab4017ba87b831f9a87b95)