debian: drop Splash= logo, it's PNG but ukify wants BMP

Author: bluca

mkosi.profiles/desktop/mkosi.conf.d/debian/mkosi.conf

@@ -4,7 +4,6 @@
 Distribution=debian
 
 [Content]
-Splash=/usr/share/pixmaps/debian-logo.png
 Packages=
         debconf
         desktop-base