How do I remove obsolete Network entries from Network Connection Applet

have you tried to delete the files in the directory /etc/NetworkManager/system-connections/ ?

You should have 1 file for every net you have tried to connect, open a terminal and use the commands:

sudo ls -l /etc/NetworkManager/system-connections/

To list all the files, after you have found the network that you want to delete, remove them with the command:

sudo rm /etc/NetworkManager/system-connections/NETWORK_NAME

You can delete connections using nmcli con delete (CONNECTION_NAME)

To delete all connection never used ( no root needed )

nmcli --fields UUID,TIMESTAMP-REAL con show | grep never |  awk '{print $1}' | while read line; do nmcli con delete uuid  $line;    done