Removed embedded profile from png

libpng version starting from 1.6 started complaining about embedded profile. It was only producing a nag screen for the use,r without any impact on usability. So after getting tired of it, I removed it as per https://wiki.archlinux.org/index.php/Libpng_errors.

Everything works fine, and no nag screen anymore. Only change is really running mogrify -strip on all pngs in src/icons.