MAINTAINERS: remove OXNAS entry

Due to lack of maintenance and stall of development for a few years now,
and since no new features will ever be added upstream, remove MAINTAINERS
entry for OXNAS files.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20230814-topic-oxnas-upstream-remove-v3-3-e2ba579a49d3@linaro.org


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>