swr: [rasterizer core] Remove dead code Clipper::ClipScalar()

swr: [rasterizer core] Remove dead code Clipper::ClipScalar()

Clipper::ClipScalar() is dead code and should be removed.  It is causing
an error with gcc-7 because it references a now defunct member.

v2: includes bugzilla reference, same code change

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99633
CC: "13.0 17.0" <mesa-stable@lists.freedesktop.org>
Tested-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>