package/mplayer: remove package

This package causes numerous build errors. During the last hackaton it
was decided to remove this package:
http://lists.busybox.net/pipermail/buildroot/2018-April/217514.html

Fixes
http://autobuild.buildroot.net/results/bda/bda461391c9e02e313fda887b75099e619527a7a/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>