following this thread, Increasing ip_conntrack_max safely? I noticed that I use ftp_conntrack, so I decided to remove ip-conntrack. However it seems it is used by 2-3 other modules, including xt_state and iptables. any idea if I can remove it anyway?
No, you can not remove modules which have dependent modules loaded. You can disable iptables, and remove the other modules that use ip-conntrack. Basically, you will be severely crippling your firewalling. ip-conntrack is one of the core modules that enable iptables to be a stateful firewall.
ftp_conntrack likely also depends ip-conntrack which handles connection tracking most of your connections. I would expect it is ip-conntrack that tracks the control channel for your FTP connections.