Rename $file['remove'] to $file['delete']

In preparation of letting imagefield use the "filefield API", this patch renames 'remove' to 'delete' (also in the user visible strings). The plan is to unify the structure of the $file array, which I intend to do by using $file['delete'] instead of $file['remove'] in filefield, and instead of $file['flags']['delete'] in imagefield. When both are done, $file arrays can be passed between filefield and imagefield functions without any doubts that it might not work.

Applies to plain HEAD.