fs: Remove unused branches

a few places the code was refactored to use `maybeCallback` which
always returns a function. Checking for `if (callback)` always
returns true anyway.

PR-URL: #5289
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: thefourtheye <thechargingvolcano@gmail.com>

lib/fs.js

@@ -1098,7 +1098,7 @@ function writeAll(fd, buffer, offset, length, position, callback_) {
   fs.write(fd, buffer, offset, length, position, function(writeErr, written) {
     if (writeErr) {
       fs.close(fd, function() {
-        if (callback) callback(writeErr);
+        callback(writeErr);
       });
     } else {
       if (written === length) {
@@ -1131,7 +1131,7 @@ fs.writeFile = function(path, data, options, callback_) {
   var flag = options.flag || 'w';
   fs.open(path, flag, options.mode, function(openErr, fd) {
     if (openErr) {
-      if (callback) callback(openErr);
+      callback(openErr);
     } else {
       var buffer = (data instanceof Buffer) ? data : new Buffer('' + data,
           options.encoding || 'utf8');