Remove a hangover from before we had Ninject wired in

I'm a bit confused by this. I can only think that I don't understand how Ninject's convention-based binding works.

I appear to need a binding for:

`IMailHostSettingsProvider` -> `SmtpHostSettingsProvider`

but not for:

`IMessageFactory` - > `SendGridMessageFactory`
`IEmailSender` -> `SendGridEmailSender`
`IAppSettingsProvider` -> `WebConfigurationAppSettingsProvider`

I don't understand why the difference.