remove go modules

Perhaps one day we'll revisit this, but right now supporting go modules is just a headache.
Dep does everything we need and it's really easy to work with, and given that supporting both simultaneously is too cumbersome, and I'm too lazy to make the switch to go modules properly, I'm sticking with go dep for now.

.circleci/config.yml

@@ -6,19 +6,6 @@ jobs:
     working_directory: /go/src/github.com/jesseduffield/lazygit
     steps:
       - checkout
-      - run:
-          name: Ensure go.mod file is up to date
-          command: |
-            export GO111MODULE=on
-            rm go.sum
-            mv go.mod /tmp/
-            go mod init
-            export GO111MODULE=auto
-
-            if [ $(diff /tmp/go.mod go.mod|wc -l) -gt 0 ]; then
-              diff /tmp/go.mod go.mod
-              exit 1;
-            fi
       - run:
           name: Run gofmt -s
           command: |