When you remove a dep from the
mix.exs file it will remain in the
mix.lock file. Unused deps need to be "unlocked" in order to be removed from the lock file.
mix deps.clean --unused
will only remove the unused dependencies' files, but will keep the library in the
mix.lock file. If you run the clean command the
--unlock option, it will also remove it from the lock file.
mix deps.clean --unlock --unused