build-support/appimage: remove libpng

libpng12 is old, vulnerable to numerous CVEs, and is
also not expected to be present on the host system [1].
Changes to the appimage FHS environment are sadly not
very testable, because it is unfeasible to run all
appimages in and out of tree. If this causes problems,
we may have to readd `libpng` (ideally the 1.6 variant
instead of the 1.2 variant). If anything specifically
requests `libpng12.so` we have a problem.

[1] 19e30b276f/excludelist

pkgs/build-support/appimage/default.nix

@@ -209,7 +209,6 @@ rec {
         flac
         libglut
         libjpeg
-        libpng12
         libpulseaudio
         libsamplerate
         libmikmod