ci(refactor): deprecate Omnibus

[boquan-fang]

Description of changes:

This PR deletes the buildspec_omnibus.yml file.

The s2nOmnibus CodeBuild job's coverage can be covered by individual jobs in our CI. We will run s2nGeneralBatch, Valgrind, s2nFuzzBatch, and AddressSanitizer instead to test our release in the future.

We are using some previous PRs to gradually drop s2nOmnibus coverages, and those PRs are PR#4913, PR#4945, and PR#4946. Such gradual drops of Omnibus' jobs can ensure that we are not losing any coverage along the way.

I have compared the rest of buildspec_omnibus.yml against the buildspec_generalbatch.yml, and found out that buildspec_generalbatch.yml covers all test jobs in the remaining buildspec_omnibus.yml. The release Lambda has also drop s2nOmnibus in the codebuild jobs list. Hence, we can fully deprecate the s2nOmnibus job now.

Call-outs:

• This PR can be merged after PR#4950 is merged. Without PR#4950, s2nGeneralBatch triggered by the Lambda will failed, because of using the wrong directory.

• The s2nOmnibus job should be removed from the Codebuild.

Testing:

• This PR will be tested by the CI.
• I will test the Lambda and attach the triggered test result to this PR: s2nGeneralBatch, S2nIntegrationV2Batch2, Valgrind, s2nFuzzBatch, and AddressSanitizer.
• I have checked and confirm that s2nGeneralBatch can provide full coverage for the rest of Omnibus.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.