Drop XFree86Server from execmem.c

Drop XFree86Server from execmem.c

This is a remnant from the libcwrapper days that can be removed now. It is the
last XFree86 glitch in Mesa core.

Reverts the following commit (thanks to Brian Paul for pointing to it):

commit 8b1dc68662f2a18ea4e02c6d3247c5d1da741aa8
Author: Alan Hourihane <alanh@tungstengraphics.com>
Date:   Tue Jan 24 21:40:53 2006 +0000

    pull in fix from 6.4 branch for XFree86Server definition

commit cc7b6810e2fe6ab485fd96d5f17bf5d9329bd153
Author: Ian Romanick <idr@us.ibm.com>
Date:   Fri Oct 21 18:09:24 2005 +0000

    Make execmem.c compile in the server (libGLcore).  Since xf86mmap
    doesn't know about MAP_ANONYMOUS, this is the only viable fix.  This
    issue will likely have to be revisited at some point.