Proposal for removing legacy code update services

[Lei YU]

Hi All,

With 3ba97559e, OpenBMC supports PNOR code update by
org.open_power.Software.Host.Updater.service, with the steps described in
[host-code-update.md][1]

And before that OpenBMC already supports BMC code update by
xyz.openbmc_project.Software.BMC.Updater.service as well, with the steps
described in [ubi-code-update.md][2]

So it's time to remove the legacy services that could be replaced:
- org.openbmc.control.BmcFlash.service
- org.openbmc.control.Flash.service
- org.openbmc.managers.Download.service

What do you think?

Thanks!

[1]: https://github.com/openbmc/docs/blob/master/code-update/host-code-update.md
[2]: https://github.com/openbmc/docs/blob/master/code-update/ubi-code-update.md