Remove dependency on gnatprove in the top-level alire.toml

[Daniel King]

The top-level alire.toml for libkeccak should not depend directly on gnatprove

The recommended workflow is to have a nested crate that depends on libkeccak and on gnatprove, that is used for the verification. This nested crate needs not to be explicitly published, but can be distributed normally with the main library sources. See here: https://github.com/alire-project/alire/blob/master/doc/catalog-format-spec.md#using-pins-for-crate-testing