Require MSBuild for libs, but keep netcore tests

build.cake

@@ -48,9 +48,6 @@ var CI_TARGETS = new string[] { "CI", "WINDOWS-CI", "LINUX-CI", "MAC-CI" };
 var IS_ON_CI = CI_TARGETS.Contains (TARGET.ToUpper ());
 var IS_ON_FINAL_CI = TARGET.ToUpper () == "CI";
 
-// temporary flag while we wait for v5.0 to become everywhere
-var USE_MSBUILD = bool.Parse (EnvironmentVariable ("USE_MSBUILD") ?? "False");
-
 string ANDROID_HOME = EnvironmentVariable ("ANDROID_HOME") ?? EnvironmentVariable ("HOME") + "/Library/Developer/Xamarin/android-sdk-macosx";
 string ANDROID_SDK_ROOT = EnvironmentVariable ("ANDROID_SDK_ROOT") ?? ANDROID_HOME;
 string ANDROID_NDK_HOME = EnvironmentVariable ("ANDROID_NDK_HOME") ?? EnvironmentVariable ("HOME") + "/Library/Developer/Xamarin/android-ndk";

cake/UtilsManaged.cake

@@ -39,22 +39,14 @@ var RunDotNetCoreRestore = new Action<string> ((solution) =>
 
 var RunMSBuildWithPlatform = new Action<FilePath, string> ((solution, platform) =>
 {
-    if (USE_MSBUILD) {
-        MSBuild (solution, c => { 
-            c.Configuration = "Release"; 
-            c.Verbosity = VERBOSITY;
-            c.Properties ["Platform"] = new [] { platform };
-            if (!string.IsNullOrEmpty (MSBuildToolPath)) {
-                c.ToolPath = MSBuildToolPath;
-            }
-        });
-    } else {
-        DotNetBuild (solution, c => { 
-            c.Configuration = "Release"; 
-            c.Verbosity = VERBOSITY;
-            c.Properties ["Platform"] = new [] { platform };
-        });
-    }
+    MSBuild (solution, c => { 
+        c.Configuration = "Release"; 
+        c.Verbosity = VERBOSITY;
+        c.Properties ["Platform"] = new [] { platform };
+        if (!string.IsNullOrEmpty (MSBuildToolPath)) {
+            c.ToolPath = MSBuildToolPath;
+        }
+    });
 });
 
 var RunMSBuildWithPlatformTarget = new Action<FilePath, string> ((solution, platformTarget) =>