printers: Do not authenticate SMB servers when search field is edited

When the search field in the "Add Printer" dialog is edited, no authentication with a remote SMB server should be attempted. The server should just be added to the list and marked with "Server requires authentication". The user can then authenticate the server by clicking on it.

The behavior described above is probably the intended one when the code was written, since there is no closure registered for the "authentication-required" signal in the function this commit modifies. This commit should simply restore the intended behavior.

Resolves: #755 (closed)