Revert "duneci-build-docker-image: set DUNECI_PARALLEL when building images"

This reverts commit e0e38897.

bin/duneci-build-docker-image

@@ -13,7 +13,7 @@ docker-build() {
 }
 
 docker_opts=()
-for var in DUNECI_PARALLEL ftp_proxy FTP_PROXY http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY; do
+for var in ftp_proxy FTP_PROXY http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY; do
   if [[ -n "${!var:-}" ]]; then
     docker_opts+=(--build-arg="${var}=${!var}")
   fi