Remove unused fork implementations

[Roland McGrath]

> On Tue, 24 Apr 2012, Roland McGrath wrote:
> 
> > It's still possible to configure for *-linux-gnu without nptl
> > (given --disable-sanity), in which case those files would be used.
> 
> The sysdeps/unix files still wouldn't be used because fork is in 
> sysdeps/unix/sysv/linux/syscalls.list, which will take precedence.  

Yes, I'm fine with removing those that can never be used.