tools: remove useless function declaration

Backport-PR-URL: #17788
PR-URL: #17125
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

tools/doc/preprocess.js

@@ -10,11 +10,7 @@ var includeData = {};
 
 function preprocess(inputFile, input, cb) {
   input = stripComments(input);
-  processIncludes(inputFile, input, function(err, data) {
-    if (err) return cb(err);
-
-    cb(null, data);
-  });
+  processIncludes(inputFile, input, cb);
 }
 
 function stripComments(input) {