Conductor Andris Nelsons announced Thursday he is cancelling his Symphony Hall debut with the Boston Symphony Orchestra, scheduled for Jan. 5, 6, and 7, citing the expected arrival of his and his wife’s first child. Nelsons’s appearance in Boston had been eagerly anticipated, as the conductor is rumored to be an unofficial candidate to replace former music director James Levine, whose resignation went into effect in September. It was the second such disappointment in less than a month, as conductor Riccardo Chailly withdrew from his two weeks of scheduled performances in January, for health reasons.