Remove "default" image description

When no image description is filled in, Pleroma allowed fallbacks.
Those were (based on a setting) either the filename, or a fixed description.
Neither are good options for image descriptions imo, so here we remove this.

Note that there's two tests removed who supposedly tested something else.
But examining closer, they didn't seem to test what they claimed to test,
so I removed them rather than try to "fix" them.