sygusComp2018: simplify beta reduction in uf rewriter. (#2106)

sygusComp2018: simplify beta reduction in uf rewriter. (#2106)

This is PR 8/18 from the sygus comp 2018 branch (https://github.com/ajreynol/CVC4/tree/sygusComp2018-2).

I am not sure how it is possible in any circumstance that the complication in the comment I am deleting would ever happen, without doing something very strange. I think it is referring to some sort of inter-dependence between macro expansion + beta-reductions. This should not concern the rewriter. Here is the commit that introduced it: bdaa304.