gating: remove gating.yaml entirely

Only `osci.brew-build.tier0.functional` was specified. By removing the
gating configuration the old 'dashboard gating' is bypassed entirely.

The default cases will still be run, including osci.tier0 and leapp.

Resolves: RHEL-111405

gating.yaml

@@ -1,6 +0,0 @@
---- !Policy
-product_versions:
-  - rhel-9
-decision_context: osci_compose_gate
-rules:
-  - !PassingTestCaseRule {test_case_name: osci.brew-build.tier0.functional}