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.