Take a look at admin/include/functions_upload.inc.php in function add_uploaded_file() line 197 and following, starting with the comment
// upload directory hierarchy

However, to get rid of the upload date altogether you would have to not only strip the subdirectories from $upload_dir but also substitute the $date_string in $filename_wo_ext (with what?), and then implement a mechanism to prevent filename clashes, the simple $random_string partial md5sum without the date+time and not in subdirectories will not be sufficient.

Note that path+filename are stored in the database table piwigo_images column `path`, so changing already existing path names will need adaption there as well.

Anyway, I doubt that upload function would be the only place to be adapted, there may be other places that assume the current structure, grep for upload_dir (of the $conf['upload_dir'] setting) and inspect code around, also plugins may be affected and even hardwire things. Be prepared that things may break or not work the way you'd like.

Also note that cramming all files into one directory may significantly slow down things for a huge amount of images, even worse with the thumbnail derivatives that are generated under _data/i/upload/ (however that might be affected by the change) that even multiply the amount of files.

What would that even be good for anyways? It's just the upload date+time you see.