Drop or fix darwin build for psutil

[nanliu]

psutil does not load on MacOS because we only support cpu metrics on linux

$ snapctl plugin load snap-plugin-collector-psutil
Error loading plugin:
GetMetricTypes call error : darwin not supported by plugin

Upstream gopsutil have resolved the issue, but requires cgo. At this point we need a decision whether we want to support this plugin for MacOS (which requires cgo and we can't easily generate universal binaries), or drop support and delete the darwin binary in the github release page.

cc @intelsdi-x/snap-maintainers

[mbbroberg]

@nanliu psutil is our go-to example of using Snap, which makes me lean toward fixing it. I know our CI with cgo isn't the easiest. Do we have another plugin requiring cgo?

FYI on this concern @jcooklin.

[marcin-krolik]

@mjbrender we don't have any other plugin which requires cgo

[sandlbn]

@marcin-krolik we have libvirt, and we dropped support for darwin.

[marcin-krolik]

@sandlbn thanks, was not aware of this one