src/poky:piglit: drop python3-six dependency

six was dropped as a dependency five years ago:
https://gitlab.freedesktop.org/mesa/piglit/-/commit/288e7b3b74a88e5cfa6a7fe26d52bf093e16365f

(From OE-Core rev: 915acbfb2e10a3e50d1ddc984b8e9c363dac6f78)

Signed-off-by: Guðni Már Gilbert <gudni.m.ggmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briandbootlin.com>
Signed-off-by: Richard Purdie <richard.purdielinuxfoundation.org>

Author: gudnimg