Remove false positive error log for morph targets (#24407)

# Objective

When mesh assets are prepared, an image is allocated only if the mesh
has any morph targets. When unloading the asset an error log is always
printed if that image cannot be found, even if it was never allocated.
This leads to a bunch of error messages that aren't actionable.

## Solution

Remove the error log, as we don't have enough bookkeeping to output it
correctly.

## Testing

None