package/qt5: simplify and cleanup _SITE variables

package/qt5: simplify and cleanup _SITE variables

The variable QT5_SNAPSHOTS_SITE was introduced in commit 9d5ac9698 to
fetch sources of obsolete modules. It was used to download qt5webkit
tarballs.

The variable is not referenced anymore since the qt5webkit tarballs are
downloaded from the Qt community release site.

Since the introduction of qt-5.9 in aa1c40ba3, the QT5_SITE variable
was duplicated for no good reason.

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>