Remove runtime OS X version check.

My recent update to that check broke compilation on 10.9:
https://code.google.com/p/dolphin-emu/issues/detail?id=7900

However, on further review, the check isn't actually necessary.  If the
OS X version is older than LSMinimumSystemVersion in Info.plist, the
system will generally refuse to run the binary in the first place.  You
can try to launch it via a terminal, but at that point it's the user's
problem if it crashes.