Drop ruamel.yaml

YAML format is complex and at times unsafe. ruamel.yaml struggles to provide a sane way to round-trip edit the file. We have seen a lot of breaking bugs related to ruamel.yaml editing. Currently it is being used in two cases. First is in tahoe-lafs and a minor reading of value in matrix-synapse. I believe we can drop the use of ruamel.yaml entirely and just edit configuration builtin yaml library. This means we will loose some formatting, comments, etc. in configuration files, but this is better than what we faced in the past.