The edit command of imag-diary gets removed by this patch.
The rationale is that imag-edit is way more powerful and thus, not every
other imag module should implement an edit command. Chaining
`imag-diary list` and `imag edit -I` is encouraged.
We still need the edit functionality in imag-diary, as `imag diary
create` shall spawn an editor, still. We only remove the ability for
editing of existing entries with this patch.
