[kalman_2] Remove redundant axhline plotting

[suda-yuga]

Remove redundant ax.axhline call outside the simulation loop.

• The true effort level (u_0) lines are already plotted inside the simulate_workers function.
• The external ax.axhline call used u_0, which was undefined outside the loop.
• This cleanup avoids redundant code and potential NameError.

[mmcky]

thanks for this PR @suda-yuga