MAINTAINERS: remove myself as staging FBTFT maintainer

Even though I did introduce the fbtft code in staging a while ago to
stop seeing this being developed out-of-tree, I don't intend to
maintain it, and I don't use it actively. So be honest and remove
myself from the MAINTAINERS file for this subsystem.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>