trace looks like it's supported but it's not

[perrydv]

I think trace snuck into the code base as a half-supported idea (presumably matrix trace) at some point. It is included in matrixSquareReductionOperators which makes it not trigger a warning when a nimbleFunction is defined, but it then fails in compilation. Flagging this as an issue so we can do a more thorough check and potentially clean it up.

[paciorek]

I'm simply removing it from matrixSquareReductionOperators. No mention of trace appears in the manual.