Remove Windows support completely for now

Since the managed libraries are built on Windows when packaging, the
released nuget would get a Mono.Unix.dll which would attempt to look
up `libc` symbols in `msvcrt`, which would break.  Since we currently
don't support Windows at all, remove support for `msvcrt`

Bump version to 7.1.0

Mono.Unix.Common.props

@@ -15,8 +15,4 @@
     <StrongNameKeyId>Open</StrongNameKeyId>
     <LangVersion>latest</LangVersion>
   </PropertyGroup>
-
-  <PropertyGroup Condition=" '$([MSBuild]::IsOSPlatform(Windows))' == 'false' ">
-    <DefineConstants>$(DefineConstants);FORCE_USE_LIBC_NOT_MSVC</DefineConstants>
-  </PropertyGroup>
 </Project>

eng/Versions.props

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
-    <VersionPrefix>7.0.1</VersionPrefix>
+    <VersionPrefix>7.1.0</VersionPrefix>
 <!--    <PreReleaseVersionLabel>final</PreReleaseVersionLabel>
     <PreReleaseVersionIteration>1</PreReleaseVersionIteration> -->
   </PropertyGroup>

src/Mono.Unix/Mono.Unix.Native/Stdlib.cs

@@ -384,11 +384,7 @@ static XPrintfFunctions ()
 	//
 	public class Stdlib
 	{
-#if FORCE_USE_LIBC_NOT_MSVC
 		internal const string LIBC = "c";
-#else
-		internal const string LIBC = "msvcrt";
-#endif
 		internal const string MPH  = "Mono.Unix";
 
 		// It is possible for Mono.Posix and Mono.Unix to get out of sync,