MAINTAINERS: mtd/ubi/ubifs: Remove inactive maintainers

Despite their substantial personal investment in the MTD/UBI/UBIFS a
few years back, David, Brian, Artem and Adrian are not actively
maintaining the subsystem anymore. We warmly salute them for all the
work they have achieved and will of course still welcome their
participation and reviews.

That said, Marek retired himself a few weeks ago quoting Harald [1]:

        It matters who has which title and when. Should somebody not
        be an active maintainer, make sure he's not listed as such.

For this same reason, let’s trim the maintainers list with the
actually active ones over the past two years.

[1] http://laforge.gnumonks.org/blog/20180307-mchardy-gpl/

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Artem Bityutskiy <dedekind1@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Artem Bityutskiy <dedekind1@gmail.com>