book: Drop crate features page

This is covered by the uefi/uefi-services crate documention. Drop the
list from here so we don't have to worry about keeping it in sync.

Author: nicholasbishop

book/src/SUMMARY.md

@@ -7,7 +7,6 @@
   - [Running in a VM](tutorial/vm.md)
 - [How-to](how_to/introduction.md)
   - [Using Protocols](how_to/protocols.md)
-  - [Crate Features](how_to/crate_features.md)
 - [Concepts](concepts/introduction.md)
   - [Boot Stages](concepts/boot_stages.md)
   - [Tables](concepts/tables.md)