Remove duplicate linters and use gci for imports

[anuraaga]

Randomly was reading through this repo and found this potential cleanup, feel free to reject if not needed

• gofmt and gofumpt fully overlap, so remove gofmt linter since gofumpt is more advanced
• gci and goimports fully overlap, so remove goimports linter since gci is more avanced.
  • If wanting to use goimports, can do the reverse, but I don't recommend it since it formats much worse IMO
• Replace goimports with gci in Makefile to correspond to above change
  • Removes awk hack since not needed for gci
  • Should be reverted if picking goimports instead of gci, basically both Makefile and golangci.yml should define the same thing

A change that came to mind but was larger than the above was replacing manual invocation of gofumpt and gci in Makefile with golangci-lint run --fix, which works very similarly because it is configured for the two. I personally like it since it means no need to reproduce configuration from yaml file into Makefile (notably the local import prefix). The downside is it would be a bit slower, so also understand why not everyone would want to do that.

[mathetake]

Thanks!