By Cecile Mantovani and Denis Balibouse LEYSIN, Switzerland (Reuters) – Deprived of snow by unusually mild weather, Switzerland’s world-renowned ski slopes have proven a disappointment to winter sports aficionados and resort managers eager to make the most of the holiday season. Recent temperatures climbing as high as a record 20.9 Celsius (69.6 Fahrenheit) have severely disrupted operations at many ski resorts, even forcing some to temporarily close. In Leysin, a resort village in the Vaud Alps, only a handful of skiers could be seen on Wednesday descending a slope where a narrow pass of arti…