Remove BuildRequires: xorg-x11-server-devel
    
    This doesn't seem to have ever been _really_ needed? This would build
    something vaguely like the modesetting X driver but targetting exa
    instead of glamor. I don't think we've ever built that driver from
    mesa.spec; we used to build xorg-x11-drv-vmware from here, back in the
    7.11 era, but that's long since been split apart. The upstream code and
    dependency on xorg-server.pc has been gone since:
    
        commit 69b425efae189af6068b28d1da5a93d27c5315a7
        Author: Matt Turner <mattst88@gmail.com>
        Date:   Sat Oct 26 17:25:11 2013 -0700
    
    	st/xorg: Delete.
    
    So that's nice, because it eliminates a nasty bootstrap issue where
    xorg-x11-server-devel requires mesa-libGL-devel.