Remove if defined? Rails check for requiring engine

MSP-11359
Fix requires spacing

MSP-11359
Fix typo

MSP-11359
Remove Metasploit::Model::Configured

MSP-11359

`Metasploit::Model::Configured` and `Metasploit::Model::Configuration`
was only needed to fake the `Rails::Engine` setup when `Rails` wasn't
being used, but now `Rails` is a hard requirement for
`metasploit-model`, so `Metasploit::Model::Configured` and
`Metasploit::Model::Configuration` can be removed.  Removing those
classes meant the code loading reverted to the default `Rails::Engine`
setup, so `lib` is no longer autoloading, meaning that Kernel#autoload
and Kernel#require needed to be used to load from `lib` now.
`Metasploit::Model::Configured` also defined `Metasploit:Model.root`.
With it gone, consumers should use `Metasploit::Model::Engine.root`.