THRIFT-2746: compiler/cpp/README.md updated

Client: Build
Patch: Konrad Grochowski

Haskell compiler no longer need to be disabled when building with mingw
diff --git a/compiler/cpp/README.md b/compiler/cpp/README.md
index 017617f..0e853fb 100644
--- a/compiler/cpp/README.md
+++ b/compiler/cpp/README.md
@@ -20,11 +20,9 @@
 ### Cross compile using mingw32 and generate a Windows Installer with CPack
 
     mkdir build_mingw32 && cd build_mingw32
-    cmake -DCMAKE_TOOLCHAIN_FILE=../../../contrib/mingw32-toolchain.cmake -DTHRIFT_COMPILER_HS=OFF ..
+    cmake -DCMAKE_TOOLCHAIN_FILE=../../../contrib/mingw32-toolchain.cmake ..
     cpack
 
-Haskell uses C++ features not supported by mingw32 such as auto or nullptr, that's why we disable it.
-
 ## Build on windows
 
 In order to build on windows a few additional steps are necessary: