Fix icon install location, remove conditional

Created by: jonathonf

This switches the icon directory installation location to /usr/share/pixmaps which is more compliant with https://specifications.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html#directory_layout .

It also removes the conditional which is checked for each application where it will only ever succeed for one; this specific change can be accomplished in the "customisation" section after the desktop launcher has been created.