qt: drop unnecessary `ClickLineEdit` widget

This widget has been a mere shadow of its former self even since
30018757 left it just wrapping a call
to `QLineEdit::setPlaceholderText()`. Considering this, and that it's only
used in one place anyway, it's really not worth keeping.

I've cleaned up related copyright notices. They had mistakenly been left in
the customwidgets files by e14f4778, and
having reviewed the history of `SearchLineEdit` there seems to be no
relevant link to the old `ClickLineEdit` code requiring these notices being
kept in the searchlineedit files.