Strip away copyrighted ICC profile

While checking for copyrighted ICC profiles, under non free licence
with a command like:

	find . -regextype posix-extended -iregex '.*\.(jpg|png)' \
		-exec sh -c 'identify -verbose "$0" \
		| grep -i copyright && echo "$0"' {} \;

The two following files have been spotted, and the ICC profiles has
been removed:

	exiftool -icc_profile"-<=" plugin/chrome/content/img/{bad,error}.png

The images have been checked identical to their source, and in addition,
the files are about 3 kB smaller without these useless binary data.
Merge pull request #120 from DavidPrevot/icc-free

Strip away copyrighted ICC profile