Remove CMake workaround for Bullet library building.

The workaround is not needed anymore as the offending code has been patched.
[ci only: master] [skip appveyor]

Source/ThirdParty/Bullet/CMakeLists.txt

@@ -24,10 +24,8 @@
 set (TARGET_NAME Bullet)
 
 # Workaround for MinGW 6.1.0 and above where it throws ICE (internal compilation error) when -O3 is used
-# Also suppress the compilation error caused by narrowing conversion from double to float inside {}
 if (MINGW AND (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 6.1.0 OR CMAKE_CXX_COMPILER_VERSION VERSION_EQUAL 6.1.0))  # 6.1.0 is the last known bad version
     string (REPLACE -O3 -O2 CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE}")
-    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-narrowing")
 endif ()
 
 # Define source files