Remove unused assets

[cooljeanius]

Makes some major progress on #57, although idk if it'll be enough to close it yet...

[cooljeanius]

Hm, looks like at least part of the report (that certain Saurian sprites were unused) was wrong:

20240825 23:17:17 error image: could not open image 'units/saurians/youths/young-ne-bob1.png'
20240825 23:17:17 error image: could not open image 'units/saurians/youths/young-ne-bob2.png'
20240825 23:17:17 error image: could not open image 'units/saurians/youths/young-ne-bob3.png'
20240825 23:17:17 error image: could not open image 'units/saurians/youths/young-ne-bob4.png'
20240825 23:17:50 error image: could not open image 'units/saurians/youths/devotee-se-bob1.png'
20240825 23:17:50 error image: could not open image 'units/saurians/youths/devotee-se-bob2.png'
20240825 23:17:51 error image: could not open image 'units/saurians/youths/devotee-se-bob3.png'
20240825 23:17:51 error image: could not open image 'units/saurians/youths/devotee-se-bob4.png'

It may be worth revisiting this, although probably only after czyzby/wesnoth-png-optimizer#7 is fixed...