Remove unnecessary require of fileutils

Today I updated my ruby core clone, and I run the usual steps to compile
ruby. When running "make", I run into the following error:

```
(... stuff ...)
encdb.h unchanged
Traceback (most recent call last):
	5: from ./tool/mkconfig.rb:19:in `<main>'
	4: from ./tool/mkconfig.rb:19:in `require'
	3: from /home/deivid/Code/ruby/lib/fileutils.rb:4:in `<top (required)>'
	2: from /home/deivid/Code/ruby/lib/fileutils.rb:4:in `require'
	1: from /home/deivid/Code/ruby/rbconfig.rb:11:in `<top (required)>'
/home/deivid/Code/ruby/rbconfig.rb:13:in `<module:RbConfig>': ruby lib version (2.5.0) doesn't match executable version (2.6.0) (RuntimeError)
uncommon.mk:780: recipe for target '.rbconfig.time' failed
make: *** [.rbconfig.time] Error 1
```

Apparently, the script that generates the root `rbconfig.rb` file requires
`fileutils`, which in turn requires 'rbconfig' (for mjit-headers it says in a
comment), which uses the `rbconfig.rb` config file in the root folder if it
exists. In my case, this file existed but had been generated on 2.5.0, thus
causing the error.

I think we can avoid this sort of circular dependency by not requiring
`fileutils`, since it does not seem to be used anywhere in the
`tool/mkconfig.rb` script since r55338.

[Fix GH-2045]

From: David Rodríguez <deivid.rodriguez@riseup.net>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66301 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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