Remove blank `Imports:` from `DESCRIPTION`

This fixes the bug which makes a blank line keep re-appearing in the `DESCRIPTION` file when you run `document()`.

I presume this wouldn't happen if we actually put content in this field, but we can reinstate it once we need it.

This issue might possibly be related to hadley/devtools#383 or hadley/devtools#394, but I'm not certain.  In any case, this workaround makes it go away.