[ci] Reduce appveyor to one MSVC Python build

Building once with a different compiler on a different platform is
useful, but running multiple builds for languages I'm not working
on is a waste of donated CPU time, and having CI reported as failing
because the Python builds on cygwin aren't currently working is
downright unhelpful.

appveyor.yml

@@ -1,5 +1,4 @@
 platform:
-- x86
 - x64
 
 environment:
@@ -7,38 +6,11 @@ environment:
     MAKEJOBS: 2
 
   matrix:
-  - SWIGLANG: csharp
-    VSVER: 12
-  - SWIGLANG: csharp
-    VSVER: 14
-  - SWIGLANG: java
-    VSVER: 14
-  - SWIGLANG: python
-    VSVER: 14
-    VER: 27
-  - SWIGLANG: python
-    VSVER: 15
-    VER: 38
-    PY3: 3
-    APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
   - SWIGLANG: python
     VSVER: 16
     VER: 39
     PY3: 3
     APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
-  - SWIGLANG: python
-    OSVARIANT: cygwin
-  - SWIGLANG: java
-    OSVARIANT: mingw
-    APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
-  - SWIGLANG: python
-    OSVARIANT: mingw
-    WITHLANG: python
-    VER: 37
-    PY3: 3
-    APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
-  - BUILDSYSTEM: cmake
-    VSVER: 14
 
 #matrix:
 #  allow_failures: