e2mmap benchmark is not working (missing gem) in Ruby 3.0.x

Hi there,

At the moment this benchmark is not working:

```
➜ fast-ruby git:(master) ✗ ruby -v code/general/raise-vs-e2mmap.rb
ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-darwin20]
:85:in `require': cannot load such file -- e2mmap (LoadError)
from :85:in `require'
from code/general/raise-vs-e2mmap.rb:2:in `'
```

The solution would be to add this to the Gemfile:

```
gem 'e2mmap'
```

It seems that that benchmark might not be wanted in this project. After all, it is comparing Ruby vs. using the e2mmap gem. See: https://github.com/JuanitoFatas/fast-ruby/issues/95

I'm going to submit a PR removing that scenario, which should fix this issue. 😄