Remove libbeat/outputs/transport

[urso]

The libbeat/outputs/transport package has been superseded by the libbeat/common/transport package, with the common/transport package being supposed to have sub-packages per use-cases/service. The libbeat/output/transport package mostly aliases libbeat/common/transport/tlscommon and should be removed.

[ycombinator]

Thanks for creating this issue! I ended up doing much of this work as part of #16150. I will pull it out of that very large PR and make a separate PR for it. Then #16150 can simply depend on the new, smaller PR.