CI: Use the correct link to our registry

The :5000 is now unrequired, and is not present in some runners list of
authorized images

.gitlab-ci.yml

@@ -2,7 +2,7 @@ stages:
     - build
 
 build-debian:
-    image: registry.videolan.org:5000/vlc-debian-unstable:20181128113529
+    image: registry.videolan.org/vlc-debian-unstable:20181128113529
     stage: build
     tags:
         - debian
@@ -26,7 +26,7 @@ build-macos:
         - make -j $(getconf _NPROCESSORS_ONLN)
 
 build-win64:
-    image: registry.videolan.org:5000/vlc-debian-win64:20181107154313
+    image: registry.videolan.org/vlc-debian-win64:20181107154313
     stage: build
     tags:
         - debian
@@ -40,7 +40,7 @@ build-win64:
         - make -j $(getconf _NPROCESSORS_ONLN)
 
 build-win32:
-    image: registry.videolan.org:5000/vlc-debian-win32:20181107151708
+    image: registry.videolan.org/vlc-debian-win32:20181107151708
     stage: build
     tags:
         - debian