Remove the 'Similar' button next to the source field in the post edit
form. Removed for multiple reasons:
* It doesn't make sense to have to open the edit form to do a reverse
image search.
* The 'Similar' button tries to redownload the file from the source,
which has various problems: the source might have been deleted, it
might have been changed or revised, it might be a format that iqdb
can't handle (ugoira/webm/mp4), or it might otherwise not match the
the actual post.
* The 'Find similar' button already exists in the sidebar and it does
the right thing by using the preview image from Danbooru, which
avoids all the above issues.
ccf32aa7a4