[TeX] \left...\right can introduce unwanted inferred mrows

This commit introduces unwanted inferred mrows into \left...\right constructions when there is more than one element within the delimiters. E.g., \left( x x \right) will end up with an mrow containing three children: the mo for the open paren, an inferred mrow containing the two mi for the x's, and the mo for the close paren.

The inferred mrow should be unpacked; just not explicit mrows. The linked commit was too aggressive, and fails to unpack these inferred rows.