blast: remove jpeg and png dependencies
blast: remove jpeg and png dependencies

brew linkage reports that blast is never linked against these libraries.
There seem to be some code in the configure script that handles jpeg and
png, but this is then used nowhere in the code.

Closes #6157.

Signed-off-by: Michka Popoff <7b0496f66f66ee22a38826c310c38b415671b832@gmail.com>

heuermh/homebrew-science,pditommaso/homebrew-science,andrewjpage/homebrew-science