Do not return 'last_checked' as part of projects array

At present, when the API is called to get the list of projects and available updates, a value of "last_checked" is appended to the end of the array.

It's not optimal to add this to the end of the array of projects as its simply a date and not a project like every other element of the returned array and means that any consumer of the API has to have special processing to remove it or skip it when processing all the projects.

Now that Site Guardian (as of 1.0.5) provides a hook "hook_site_guardian_status" where additional status information can be returned to the consumer it makes more sense to add the "last_checked" to the status values returned from it and no longer add it to the array of projects.

This will be a breaking change to any consumer relying on the value of "last_checked" but the changes are likely to be pretty straight forward.

It's worth mentioning that the "last_checked" value is NOT the last time the API is called but rather is the last time the Core update system retrieved a list of modules and available releases. Usually the core updates subsystem caches release information and will only refresh the data once every 24 hours.