foto: keep static images of height 250

Drop dynamic source replacement to match actual client sizes, as the quality
loss from downsizing a _larger_ image is minimal.

Originally introduced in lijtweg commit v2.5-7-gcd93e774e6 (2017-10-11)
[foto: replace justified thumbnails by height-optimised versions]
with visible improvements probably due to using the target size of 200px
without adjusting for up to 1.25x enlargement to fit.

Testing originals of 250px on modern browsers with (bilinear) rescaling
is hard to distinguish (not significantly more blurry) and depends on only
a single thumbnail resolution to be created for significantly reduced
resource requirements.  May be slightly larger on average, but likely much
smaller without reloads and considering caching opportunities.