ui/common: Remove unused artwork

A few of the oxygen icons are no longer used, so remove them from the
artwork tree.

Coincidentally, this fixes and issue with `make dist`, as the
drive-removable-media-usb-pendrive.png file is too long (when used with
an autogenerated git version string) to be tar-ed up.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Geoff Levand <geoff@infradead.org>