Icon redux 01: Tidy hicontrast icons

- Delete placeholder (red-X) images; it's better we fallback to the main icons from the Galaxy theme.
- Delete non-hicontrast (normal) images, duplicated from the Galaxy theme. These were introduced by the Sidebar mainly, and 2 in Report Designer.
- Lossless optimisation of png images (imageoptim.com).

QA - Test icons throughout application using the hicontrast icon theme.

Change-Id: I45256563653792538d24bc3139b303de624544c2
Signed-off-by: Andrew <dent.ace@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/8794
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>