Remove an unnecessary special case to check for a few Darwin-specific options.

Using -mmacosx-version-min (etc.) on non-Darwin platforms should be a warning,
not a hard error. There is no reason to add a special check for these options
in the default toolchain. This just removes the special check and then we get
the usual -Wunused-command-line-argument warning if someone tries to use one
of these options for a target where they are not supported.
<rdar://problem/15569346>