service: disable debug logging by default

[sillymotives]

The packaged systemd service currently launches dbus-service with --debug on every normal startup.

On a Synaptics/Validity sensor in an HP EliteBook x360 1030 G2, debug mode wrote raw sensor protocol traffic and cryptographic material, including private-key data, into the system journal. This information is not needed for normal operation and makes the default journal unnecessarily sensitive and noisy.

This PR removes --debug from the default ExecStart line. Debug mode remains available for explicit troubleshooting by running the backend manually or overriding the service when needed.
Validation

Tested on Debian 13.7 with:

python3-validity 0.14~ppa1-based packaging;
open-fprintd 0.7~ppa2-based compatibility daemon;
Synaptics/Validity sensor reported as 57K0 FM- 154-001 (type 1825).

After removing --debug:

python3-validity.service started normally;
the sensor initialized and calibration data loaded;
the backend registered /net/reactivated/Fprint/Device/0 with open-fprintd;
existing enrolled fingerprints continued to verify;
PAM fingerprint authentication passed;
GNOME lock-screen fingerprint unlock passed;
cold-boot GDM fingerprint login passed.

Scope
This PR deliberately does not change whether the service is enabled by packaging. A separate real-world issue was observed where the backend was disabled after installation and therefore absent after reboot, but the correct upstream activation policy should be evaluated separately rather than bundled into this logging fix.