Remove gitlab zombienet tests for cumulus/substrate (#7870)

Follow up of : #7217, #7529 

When the zombienet tests were integrated into github, it seems like the
gitlab ones remained active. But they look a lot more unstable. This
causes confusion when the same tests fail and pass at the same time
(aside from extra costs). This PR attempts to get rid of the gitlab
ones.

Author: skunert

.gitlab/pipeline/zombienet.yml

@@ -12,10 +12,6 @@
   timeout: 60m
 
 include:
-  # substrate tests
-  - .gitlab/pipeline/zombienet/substrate.yml
-  # cumulus tests
-  - .gitlab/pipeline/zombienet/cumulus.yml
   # polkadot tests
   - .gitlab/pipeline/zombienet/polkadot.yml
   # bridges tests