By Sam Nussey TOKYO (Reuters) -SoftBank Group Corp's decision to sell down its Alibaba Group Holding stake for a $34 billion gain may be aimed at bolstering its finances, but it also underlines how CEO Masayoshi Son has cooled on China tech. Son was formerly one of the sector's biggest cheerleaders and Alibaba is his most famous bet, immensely profitable and for his fans, symbolic of his foresight and investing acumen. Amid a sharp market downturn, however, Son will reduce his conglomerate's stake in Alibaba to 14.6% from 23.7% by settling prepaid forward contracts, although the Chinese firm remains SoftBank's…