Disable regular (non-llvm) builds to save time

because appveyor wouldn't run them in parallel. llvm builds are only ~1
min longer anyway.

appveyor.yml

@@ -1,17 +1,13 @@
 version: 1.0.{build}
 os: Visual Studio 2015
 configuration:
- - Release
+# - Release
  - ReleaseLLVM
 platform: x64
 clone_folder: c:\rpcs3
 clone_depth: 1
 test: off
 
-matrix:
- allow_failures:
-  - configuration: ReleaseLLVM
-
 branches:
  except:
    - ppu_recompiler