Problem: commit a464eec7 really broke VS2015 builds

Took me over 8 hours to track down the cause of bizarre link
errors when building with libsodium. The VS project files are
not simple things.

Note to self and other maintainers: when someone is obviously
out of their depth, do not merge their changes to build scripts
without cynical appraisal.

Solution: undo the damage.

builds/msvc/vs2015/libzmq/libzmq.vcxproj

@@ -66,14 +66,6 @@
     <Import Project="$(ProjectDir)..\..\properties\Output.props" />
     <Import Project="$(ProjectDir)$(ProjectName).props" />
   </ImportGroup>
-  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='DebugDLL|Win32'">
-    <Linkage-libsodium />
-    <Option-sodium />
-  </PropertyGroup>
-  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='ReleaseDLL|x64'">
-    <Linkage-libsodium />
-    <Option-sodium />
-  </PropertyGroup>
   <ItemGroup>
     <ClInclude Include="..\..\platform.hpp" />
     <ClInclude Include="..\..\resource.h" />