Try this:
Lua Code:
RedirectTexture("EsoUI/Art/MapPins/skyshard_seen.dds", "")
it will exchange the texture with none and thus should be empty/hide it.
If this does not work try this (an empty/blank texture):
Lua Code:
RedirectTexture("EsoUI/Art/MapPins/skyshard_seen.dds", "/esoui/art/icons/heraldrycrests_misc_blank_01.dds")
Add this line to any addon's EVENT_ADD_ON_LOADED callback function or call it with /script RedirectTexture... ingame in the chat (this would need to be repeated after each loading screen/zone change/logout then!)
Attention: This might only work properly if you use an addon and NOT the /script approach, because
if the textures do not update properly try to logout and delete the file live/shader_cache.cooked
It contains the cached textures and thus would show the old skyhards icon.