[#599] Makefile: Remove test_dep target

Flag `-i` is deprecated in go1.16 and it
has pretty much no effect since go1.10 when
build cache was introduced. Read more about
this at https://golang.org/doc/go1.16

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

.github/workflows/go.yml

@@ -35,9 +35,6 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-go-${{ matrix.go }}-
 
-      - name: Get test dependencies
-        run: make test_dep
-
       - name: Run go test
         run: go test -coverprofile=coverage.txt -covermode=atomic ./...