Remove unused cp_r preserveMode option

I had originally created a commit which let preserveMode reuse the
return value of the fs.statSync a few lines up in the file, but since
the code is dead, there was no way to test it. Removing the dead code
seemed like a better approach.  (If you want to revert this commit, I
recommend reusing the stat buffer!)

tools/files.js

@@ -281,9 +281,6 @@ files.mkdir_p = function (dir, mode) {
 // file whose basename matches one of the regexps, before
 // transformation, will be skipped.
 //
-// If options.preserveMode is preset, an attempt will be made to
-// preserve the file's mode (Unix permissions).
-//
 // Returns the list of relative file paths copied to the destination,
 // as filtered by ignore and transformed by transformFilename.
 files.cp_r = function (from, to, options) {
@@ -316,10 +313,6 @@ files.cp_r = function (from, to, options) {
       }
       copied.push(f);
     }
-
-    if (options.preserveMode) {
-      fs.chmodSync(fullTo, fs.statSync(fullFrom).mode);
-    }
   });
   return copied;
 };