github workflows: remove gofmt and goimports actions and replace with go command invocations

[rjeczalik]

Those actions are not being actively updated:

https://github.com/Jerome1337/goimports-action (last update 1 years ago)
https://github.com/Jerome1337/gofmt-action (last update 3 year ago)

Also they are not that useful and can be replaced with a single bash one-liner.

So let's do that, dropping external dependencies is always a good thing to do.

Also let's removed forked repositories:

https://github.com/scylladb/goimports-action
https://github.com/scylladb/gofmt-action