Remove pre-2.2 Linux kernel support

[Roland McGrath]

> On 05/14/2012 09:24 PM, Roland McGrath wrote:
> > testrtsig.h is still used by signal/allocrtsig.c
> 
> Let me rephrase my comment: We now have a file in sysdeps/generic where 
> kernel_has_rtsig is always 0 and a Linux specific one that always 
> returns 1. What is the right value for Hurd? If it's 0, we could cleanup 
> further...

It's 0 for Hurd.  So what's wrong with the status quo?