Remove confusing text from description form

In the forms to update or create Images, the description
field contains "Additional information here..."
which isn't particular clear (it sounds like you should
click there to see more information) and is causing
inaccurate translations.
This hint text actually does not provide any information,
comparing with other fields.
Removing it.

Change-Id: I4c223a2b995aaa73f9cfcca56516baaaf1c72831
Closes-Bug: #1251199
Merge "Remove confusing text from description form"