CI: remove deprecated linters

.golangci.yml

@@ -10,13 +10,10 @@ linters:
     # make sure all errors returned by functions are handled
     - errcheck
 
-    # find unused code
-    - deadcode
-
     # show how code can be simplified
     - gosimple
 
-    # # make sure code is formatted
+    # make sure code is formatted
     - gofmt
 
     # examine code and report suspicious constructs, such as Printf calls whose
@@ -35,12 +32,6 @@ linters:
     # find unused variables, functions, structs, types, etc.
     - unused
 
-    # find unused struct fields
-    - structcheck
-
-    # find unused global variables
-    - varcheck
-
     # parse and typecheck code
     - typecheck