Windows/VS2017: Omit WindowsTargetPlatformVersion from project files.

I understood that if a WTPV is specified, it's often wrong
because different VS installations have different SDK version
installed. Omitting the WTPV tag makes VS2017 default to
Windows SDK 8.1 which often is also missing, so in any case
people may need to specify the WTPV before building. But some
day in the future a missing WTPV tag will start to default to
the latest installed SDK which sounds reasonable:

https://developercommunity.visualstudio.com/content/problem/140294/windowstargetplatformversion-makes-it-impossible-t.html

Thanks to "dom".

windows/INSTALL-MSVC.txt

@@ -30,6 +30,10 @@ Building
     compiler switch to link to the CRT statically, so it will not
     have any other DLL dependencies.
 
+    The VS2017 project files don't set <WindowsTargetPlatformVersion>.
+    This means that you may need to either install Windows SDK 8.1 or
+    you may need to set the target platform version before building.
+
     Currently no test programs are built or run under MSVC.
 
     MSVC gives a bunch of compiler warnings. Some warnings are specific

windows/vs2017/liblzma.vcxproj

@@ -29,7 +29,6 @@
   <PropertyGroup Label="Globals">
     <ProjectGuid>{12728250-16EC-4DC6-94D7-E21DD88947F8}</ProjectGuid>
     <Keyword>Win32Proj</Keyword>
-    <WindowsTargetPlatformVersion>10.0.15063.0</WindowsTargetPlatformVersion>
   </PropertyGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="Configuration">

windows/vs2017/liblzma_dll.vcxproj

@@ -29,7 +29,6 @@
   <PropertyGroup Label="Globals">
     <ProjectGuid>{E0F247DB-EF12-4755-8DF9-F74BCD1348F7}</ProjectGuid>
     <Keyword>Win32Proj</Keyword>
-    <WindowsTargetPlatformVersion>10.0.15063.0</WindowsTargetPlatformVersion>
   </PropertyGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="Configuration">