Removing go.mod for v3 releases (golang-jwt#26)

* Removing `go.mod` for the v3-release branch

As discussed in full length here (golang-jwt#17), we have run into issues that forces us to abandon go modules, at least for the `v3.x.x` releases. After this is merged in, we can release a `v3.2.1+incompatible` version, which contains a security fix. 

Afterwards, we will work on non-breaking quality of life fixes and then eventually run a `v4` version, which most likely will then support go modules and have a new SIV-style import path.

* Cloning into $GOPATH for GitHub actions

.github/workflows/build.yml

@@ -17,6 +17,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v2
+        with:
+          path: src/github.com/golang-jwt/jwt
       - name: Setup Go
         uses: actions/setup-go@v2
         with:
@@ -26,3 +28,6 @@ jobs:
           go vet ./...
           go test -v ./...
           go build ./...
+        env:
+          GO111MODULE: auto
+          GOPATH: ${{ github.workspace }}