MAINTAINERS: update release manager

Found-by: Compn
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

MAINTAINERS

@@ -44,7 +44,7 @@ project server                          Árpád Gereöffy, Michael Niedermayer,
 mailinglists                            Michael Niedermayer, Baptiste Coudurier, Lou Logan
 presets                                 Robert Swain
 metadata subsystem                      Aurelien Jacobs
-release management                      Diego Biurrun, Reinhard Tartler
+release management                      Michael Niedermayer
 
 
 libavutil