Get rid of gaia-taskcluster

[djmitche]

This tool is currently supporting docker-worker CI. Once that use is removed, it can be disabled completely.

[djmitche]

@gregarndt I think you're working on this -- and I think we have consensus :)

[gregarndt]

Work will be tracked in https://bugzilla.mozilla.org/show_bug.cgi?id=1244105

[gregarndt]

This has been completed.

[djmitche]

This RFC is stored as rfcs/0044-Get-rid-of-gaia-taskcluster.md