chore: remove unnecessary cd in certs:add

This was used before we started using `plugn` for triggering subshell calls to dokku.

plugins/certs/subcommands/add

@@ -72,7 +72,6 @@ certs_set() {
   cp "$KEY_FILE" "$APP_SSL_PATH/server.key"
   chmod 750 "$APP_SSL_PATH"
   chmod 640 "$APP_SSL_PATH/server.crt" "$APP_SSL_PATH/server.key"
-  cd "$DOKKU_ROOT"
   plugn trigger post-certs-update "$APP"
   plugn trigger post-domains-update "$APP"
 }