Remove lib/benchmark/helpers.rb from Manifest file.

This file has already been removed from https://github.com/JuanitoFatas/benchmark-ips/commit/bc44856d5145fdebe9e07b9dbb4bc27cfacb0f6a.

`rake gem` now results in:

```
rake aborted!
Don't know how to build task 'lib/benchmark/helpers.rb'

Tasks: TOP => gem => pkg/benchmark-ips-2.0.1.gem
(See full trace by running task with --trace)
```

This commit fixes it. Sorry I forget to remove it from `Manifest.txt`.