book: remove link to nonexistent file utilities.html

src/explore/explore.md contains a link to a "Utilities" file. This file
no longer exists (it was removed in commit 2854c9b0). In order to avoid
unnecessary 404-type errors, remove the link to this page.

src/explore/explore.md

@@ -8,5 +8,4 @@ Redox is meant to be an *insanely* customizable system, allowing a user to tear
 
 - [Boot Process](./boot_process.html)
 - [Shell](./shell.html)
-- [Utilities](./explore/utilities.html)
 - [GUI](./gui.html)