In the 1874 RLDS edition, the verb was was omitted, probably unintentionally. The 1908 RLDS edition restored the was. Either reading, with or without the was, is possible in English. Elsewhere in the Book of Mormon text there is evidence for both possibilities:
In each case of “X thought it (was) expedient”, the critical text will follow the earliest sources in determining whether the was should be there or not.
Summary: Maintain the linking verb be in Alma 46:31: “Moroni thought it was expedient that …” (the reading of the earliest textual sources, including 𝓞).