Running pacman -Rn tmux, will not remove your tmux.conf file(s). If you know their location (should be /etc/tmux.conf and / or ~/.tmux.conf) just delete them and create new one(s). If you do not know the location(s) I recommend either of two options: 1) find your tmux.conf using mlocate, then delete and replace it per your needs or 2) create a new tmux.config and pass -f new.config.file.location to tmux.
1) pacman -Syu mlocate
updatedb locate tmux.conf Then either modify
tmux.conf or delete it and create a new one.
Or
2) create ~/.tmux.conf then run tmux -f ~/.tmux.conf
pacman -Rn tmuxwill delete tmux and not keep configuration files around. Note that these are system configuration files, it will not alter or delete any personal configuration files. Linux programs generally work without a personal configuration file unless it is needed to specify remote user accounts.pacman -Rc tmuxbut to no avail.