Here the 1858 Wright edition introduced the relative pronoun what in place of the original that which. The first two RLDS editions followed the Wright reading, but the 1908 RLDS edition restored the correct that which. Elsewhere in the text, we have 29 occurrences of that which was, while there is only one occurrence of what was:
There is no reason to reject the original reading here in Mormon 2:10.
Summary: Maintain the use of that which in Mormon 2:10, a very normal construction in the Book of Mormon text.