By Michelle Chen HONG KONG, March 8 (Reuters) - While offshore yuan investors have recently complained of a lack of channels to manage the renminbi they hold, they have now turned to the other extreme: worrying about the contracting yuan deposit base in Hong Kong. Those worries might prove to be for naught, given that the decline in the deposit base appears to be caused mainly by the opening of more investment opportunities and repatriation channels. Yuan deposits may be set for a