gallium/targets: remove r600/xorg

Reviewed-by: Christian König <christian.koenig@amd.com>
gallium/targets: remove radeonsi/xorg

Reviewed-by: Christian König <christian.koenig@amd.com>
gallium/targets: remove r300/xvmc

Reviewed-by: Christian König <christian.koenig@amd.com>
gallium/targets: remove r300/vdpau

Reviewed-by: Christian König <christian.koenig@amd.com>
gallium/targets: remove xvmc-softpipe

Reviewed-by: Christian König <christian.koenig@amd.com>
gallium/targets: remove vdpau-softpipe

Reviewed-by: Christian König <christian.koenig@amd.com>