CLEANUP: contrib: remove the last references to the now dead contrib/ directory

Now with the last SPOA modules gone, contrib/ doesn't exist anymore
and does not need to be referenced in the Makefile nor .gitignore.
diff --git a/CONTRIBUTING b/CONTRIBUTING
index 73a27c7..db89ef7 100644
--- a/CONTRIBUTING
+++ b/CONTRIBUTING
@@ -818,9 +818,7 @@
 
   - poll      any of the pollers
 
-  - halog     the halog sub-component in the contrib directory
-
-  - contrib   any addition to the contrib directory
+  - halog     the halog sub-component in the admin directory
 
   - htx       general HTX subsystem