WinRT: removed Windows 8.0 from build-bot + NuGet package creation scripts

According to Steam's OS stats, Windows 8.0 use is pretty much nil.  Further,
Microsoft hasn't support Windows 8.0 development in any of their
actively-updated toolchains, and setting it up can be a pain.

In theory, SDL2 still supports Windows 8.0, however building of Windows 8.0
.dlls is no longer the default, if and when using the 'winrtbuild.*' scripts.
The MSVC 2012 project files for building Windows 8.0 dlls remain, though,
for the time being.

VisualC-WinRT/SDL2-WinRT.targets

@@ -2,7 +2,6 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
   <PropertyGroup>
-    <LibSDL2-DeviceType Condition="'$(TargetPlatformIdentifier)' == 'Windows' and '$(TargetPlatformVersion)' == '8.0'">WinRT80</LibSDL2-DeviceType>
     <LibSDL2-DeviceType Condition="'$(TargetPlatformIdentifier)' == 'Windows' and '$(TargetPlatformVersion)' == '8.1'">WinRT81</LibSDL2-DeviceType>
     <LibSDL2-DeviceType Condition="'$(TargetPlatformIdentifier)' == 'Windows Phone' and '$(TargetPlatformVersion)' == '8.0'">WinPhone80</LibSDL2-DeviceType>
     <LibSDL2-DeviceType Condition="'$(TargetPlatformIdentifier)' == 'WindowsPhoneApp' and '$(TargetPlatformVersion)' == '8.1'">WinPhone81</LibSDL2-DeviceType>

build-scripts/winrtbuild.ps1

@@ -220,9 +220,15 @@ if ( ! (Build-SDL-WinRT-Variant "SDL" "v120_wp81" "ARM"))   { $DidAnyDLLBuildFai
 if ( ! (Build-SDL-WinRT-Variant "SDL" "v120_wp81" "Win32")) { $DidAnyDLLBuildFail = $true }
 
 # Build for Windows 8.0 and Windows RT 8.0, via VC++ 2012:
-if ( ! (Build-SDL-WinRT-Variant "SDL" "v110" "ARM"))        { $DidAnyDLLBuildFail = $true }
-if ( ! (Build-SDL-WinRT-Variant "SDL" "v110" "Win32"))      { $DidAnyDLLBuildFail = $true }
-if ( ! (Build-SDL-WinRT-Variant "SDL" "v110" "x64"))        { $DidAnyDLLBuildFail = $true }
+#
+# Win 8.0 auto-building was disabled on 2017-Feb-25, by David Ludwig <dludwig@pobox.com>.
+# Steam's OS-usage surveys indicate that Windows 8.0 use is pretty much nil, plus
+# Microsoft hasn't supported Windows 8.0 development for a few years now.
+# The commented-out lines below may still work on some systems, though.
+# 
+#if ( ! (Build-SDL-WinRT-Variant "SDL" "v110" "ARM"))        { $DidAnyDLLBuildFail = $true }
+#if ( ! (Build-SDL-WinRT-Variant "SDL" "v110" "Win32"))      { $DidAnyDLLBuildFail = $true }
+#if ( ! (Build-SDL-WinRT-Variant "SDL" "v110" "x64"))        { $DidAnyDLLBuildFail = $true }
 
 # Build for Windows 8.1 and Windows RT 8.1, via VC++ 2013:
 if ( ! (Build-SDL-WinRT-Variant "SDL" "v120" "ARM"))        { $DidAnyDLLBuildFail = $true }