Remove jenkins_build_snapshot test script

[epixa]

Once the task has been removed for all branches 5.0+, we should delete jenkins_build_snapshot.sh from the repo entirely.

We must keep this file around for 4.6 though, since it will continue on CI there.

See #10970

[jbudz]

Will this to remove builds entirely from kibana jenkins?

[epixa]

@jbudz The script to remove builds kibana and then uploads it to s3. We still build Kibana elsewhere in the jenkins jobs though: https://github.com/elastic/kibana/blob/master/test/scripts/jenkins_unit.sh#L8

And @solostyle was working on getting the selenium tests themselves to run on builds in jenkins, so it'll build there as well.