Darwin: Remove deployment target version check

Starting with Xcode 8 the SDK folder also contains an unversioned entry:

    MacOSX.sdk
    MacOSX10.12.sdk -> MacOSX.sdk

If this unversioned path is used CMake cannot detect the SDK version.

Furthermore, querying the SDK version via

    xcodebuild -sdk <sysroot> -version Path

gives bogus results for the Command Line Tools installed into `/`.

The OS X deployment target version and SDK version are not as tied as
they once were, so this check is now more trouble than it is worth.
Simply remove it.

Closes: #16323