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.

Fixes: #325, #324, #322, #321, #319, #318, #317, #315, #313, #311, #310,
       #306, #305, #304, #302, #301, #300, #299, #298, #297, #293, #289

Reviewed-by: Eric Anholt <eric@anholt.net> (commit message rewritten)
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <mesa/mesa!8069>