How can I remove the death counter on my server?

First, you would need to get the name of the objective. You can get the list of names of the created objectives in your world by running the /scoreboard objectives list command.
You would then remove the said scoreboard objective using the /scoreboard objectives remove <objective> command, <objective> being the name of the scoreboard objective.

If you only want the objective to not display in the sidebar or the player tab, you could run /scoreboard objectives setdisplay <slot>, <slot> being sidebar, or list