Improvement: Add method to delete pipeline

[SamerAdra]

Added in GitLab 11.6 is a method to delete a pipeline.
https://docs.gitlab.com/ee/api/pipelines.html#delete-a-pipeline

[gmessner]

@SamerAdra
This has been added and is available in release 4.9.18, please verify and close when you have a chance.

[SamerAdra]

Works like a charm. Tested against GitLab 11.8.0-ee. Thank you @gmessner.