Remove unused `isZero` func (#364)

`isZero` unused since 4e7ffd8.
diff --git a/lint.go b/lint.go
index e1a5f30..8bb1faa 100644
--- a/lint.go
+++ b/lint.go
@@ -1606,11 +1606,6 @@
 	return ok && isIdent(sel.X, pkg) && isIdent(sel.Sel, name)
 }
 
-func isZero(expr ast.Expr) bool {
-	lit, ok := expr.(*ast.BasicLit)
-	return ok && lit.Kind == token.INT && lit.Value == "0"
-}
-
 func isOne(expr ast.Expr) bool {
 	lit, ok := expr.(*ast.BasicLit)
 	return ok && lit.Kind == token.INT && lit.Value == "1"