golangci.yml: Removing inactivated linters

Removing "inactivated" linters, which are causing lint checks to fail
with:
"This linter is fully inactivated: it will not produce any reports."

The linters that have been removed are:
- deadcode
- golint
- structcheck
- varcheck

Apparently these linters were "inactivated" in v1.57.0
https://github.com/golangci/golangci-lint/blob/master/CHANGELOG.md#v1570

.golangci.yml

@@ -11,8 +11,6 @@ linters-settings:
           - (github.com/golangci/golangci-lint/pkg/logutils.Log).Warnf
           - (github.com/golangci/golangci-lint/pkg/logutils.Log).Errorf
           - (github.com/golangci/golangci-lint/pkg/logutils.Log).Fatalf
-  golint:
-    min-confidence: 0
   gocyclo:
     min-complexity: 10
   maligned:
@@ -57,19 +55,15 @@ linters:
     - ineffassign
     - stylecheck
     - misspell
-    - deadcode
     - staticcheck
-    - structcheck
     - unused
     - prealloc
     - typecheck
-    - varcheck
     # additional linters
     - bodyclose
     - gocritic
     - goerr113
     - goimports
-    - golint
     - gomnd
     - misspell
     - noctx