removeEqualFunctionCalls should be better documented and merged with comSubExp

[casella]

In the process of fixing #5458, the removeEqualFunctionCalls was temporarily disabled, which caused some problems. We also discovered that it does a lot more that the documentation says, as documented in the comments to that ticket. It was eventually reinstated with ​PR #341, causing some ​regressions.

We should improve the ​documentation of this module, or possibly merge it with comSubExp, and try to fix the regressions, assuming they are actually due to that PR.