engine: removed unused cflags from ios-simulator, fixes #32

clang complained when building for ios-simulator:

clang: warning: argument unused during compilation: `-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.0.sdk`
clang: warning: argument unused during compilation: `-miphoneos-version-min=5.0`