Remove GCE cloud setting from AppVeyor config

AppVeyor has informed us that this may no longer be necessary after some
infrastructure upgrades on their side, so let's see how this goes!

Author: alexcrichton

appveyor.yml

@@ -5,12 +5,6 @@ environment:
   # server goes down presumably. See #43333 for more info
   CARGO_HTTP_CHECK_REVOKE: false
 
-  # Recommended by AppVeyor this moves our builds to GCE which incurs a 3-4
-  # minute startup overhead, but that's paltry compared to our overall build
-  # times so we're will to eat the cost. This is intended to give us better
-  # performance I believe!
-  appveyor_build_worker_cloud: gce
-
   matrix:
   # 32/64 bit MSVC tests
   - MSYS_BITS: 64