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ARROW-2992: [CI] Remove some AppVeyor build configurations

Remove two build configurations:
- Release build with Ninja and VS2015 (we already have a Release build with
  Ninja and VS2017, and a Debug build with Ninja and VS2015)
- Release build with NMake (we don't care much about NMake enough to add
  10+ minutes of build time)

Author: Antoine Pitrou <antoine@python.org>

Closes #2388 from pitrou/ARROW-2992-remove-appveyor-jobs and squashes the following commits:

cdf246f <Antoine Pitrou> Move slow jobs earlier in the sequence
3568847 <Antoine Pitrou> ARROW-2992:  Remove some AppVeyor build configurations