Run the following command in a terminal:
gsettings get org.gnome.desktop.sound theme-name
this will print the selected sound theme.
For me this will print 'freedesktop'.
Then go to /usr/share/sounds/freedesktop/stereo (replace freedesktop with your sound theme if it’s different) and rename or delete the power-plug.oga and power-unplug.oga there. (as root)