nv50: Remove unused value

nv50: Remove unused value

Fix defect reported by Coverity Scan.

Unused value (UNUSED_VALUE)
assigned_pointer: Assigning value from &nv50->vtxbuf[b] to vb here, but
that stored value is overwritten before it can be used.

Fixes: 76725452239 ("gallium: move vertex stride to CSO")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24801>