Remove balance.png from Default skin

This icon should not be used as it makes balancer white on Default
skin which is black. File is installed with meson (not with make).
It looks like copy paste error from Classic1.3 skin.

Fixes: 00f66d5 ("Add a common skins-data folder.")

Reported-by: Jan Knezik <[email protected]>
Signed-off-by: Petr Vorel <[email protected]>

Author: pevik