net-im/folks: drop unused Meson options after 82ec8506217a

meson.build:1:0: ERROR: Unknown options: "tracker_backend"

https://gitlab.gnome.org/GNOME/folks/-/commit/e9215d339325

PR: 259080
Reported by: antoine (via exp-run)
Regressed by: https://github.com/mesonbuild/meson/commit/dbf2ace6ca1c