Removed references to non-existent Wigram Aerodrome (Christchurch, New Zealand)

Wigram Aerodrome was closed four years ago. It has been completely demolished and it being redeveloped for housing, shopping malls and a business park. 

I belive it should be removed because the location of only of historical value and showing a airport warning in Mission Planner for a non-existent airport may train local people to ignore these warnings (for airports that do exist)

Proof of the closure can be seen here:

* http://www.radionz.co.nz/news/regional/39204/historic-wigram-aerodrome-closed
* http://www.stuff.co.nz/the-press/christchurch-life/mainlander/560197/A-sense-of-closure
* http://www.wigramskies.co.nz/wigram-past-and-future/history
* http://www.3news.co.nz/Closure-means-end-of-an-era-for-Wigram-airfield/tabid/423/articleID/62475/Default.aspx
* http://www.stuff.co.nz/the-press/business/your-property/9232819/Wigram-subdivision-selling-sections
* http://www.stuff.co.nz/business/industries/9346422/New-shopping-centre-for-Wigram-Skies
* http://en.wikipedia.org/wiki/Wigram_Aerodrome
* http://ngaitahuproperty.co.nz/development/residential/wigram-skies

It was already listed as "closed" in airports.csv, but this did not prevent it from appearing in Mission Planner.