network-agent: Remove "Show password" switch

network-agent: Remove "Show password" switch

While the ability to show the password can be useful at times,
the existing implementation is problematic:

  1) The use of a switch is wrong (as even noted in a code
     comment).

  2) It is inconsistent with any other password dialog (login screen,
     polkit).

In lack of a properly designed solution (for all password dialogs),
the designers agreed to remove the switch for now.

https://bugzilla.gnome.org/show_bug.cgi?id=658948