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.

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: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8069>

meson_options.txt

@@ -45,7 +45,7 @@ option(
   'dri-drivers',
   type : 'array',
   value : ['auto'],
-  choices : ['auto', 'i915', 'i965', 'r100', 'r200', 'nouveau', 'swrast'],
+  choices : ['auto', 'i915', 'i965', 'r100', 'r200', 'nouveau'],
   description : 'List of dri drivers to build. If this is set to auto all drivers applicable to the target OS/architecture will be built'
 )
 option(