Doc: Remove nonexistent qtwebkitwidgets reference

There is no such qtwebkitwidgets.index file to be used in \depends

Change-Id: I191801d22d908721e8d65b610f684c23b7dc7870
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>