swrast: Remove the classic swrast DRI driver

Following up from the classic swrast OSMesa removal in favor of llvmpipe/softpipe, remove the classic swrast DRI driver. It's unused by any distribution -- debian was the last holdout until this week, which had it enabled instead of softpipe on obscure platforms (non-LLVM and hurd). Now that debian has switched, remove the driver so nobody can accidentally enable it again.