Do not depend on bash uncessarily, and do not validate icons by default.

Summary:
I don't think it makes sense, that the icons are xmllint'ed every time someone builds the icons.
This is an action that should take place once, before cutting the next release.

Also, from a first glance, the scripts do not actually require bash, and should work with -- let's call it 'standard' -- /bin/sh.

Reviewers: FreeBSD, winterz, tcberner

Subscribers: adridg, aacid, rakuco, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6076