Standard Sony firmware flashed via Flashtool often does not wipe user data (to allow for updates). However, "Lock Remove" FTF files are modified specifically to include the userdata.sin partition in the flashing process, or they are configured to force a data wipe.
using an FTF file, you can flash the original stock firmware using the Flashtool . This process performs a factory reset and removes all user settings, including the lock screen. Prerequisites PC with Flashtool installed. Drivers: Install Sony Flash Device drivers sony c6802 lock remove ftf