Use a better and simpler default name for multi-file archives

Now the default name of a multi-file archive created from the file
actions plugin (i.e. from Dolphin's context menu) will be
"Archive.[extension]", not "[name of parent folder].[extension]". That
was a weird default filename choice.

This partially reverts 77606102, which
added a function that is no longer needed; that function has been
removed.

CCBUG: 446728