Remove .is_infinitesimal assumption

See discussion in #7943, e.g. [this](https://github.com/sympy/sympy/issues/7943#issuecomment-54041566).

fixes #5263
I don't *think* we are losing any functionality in getting rid of this. You can always make var('eps', complex=True) and then look at the behavior as eps -> 0 for a disred direction. But we aren't using this and until we have a use case I am in favor :+1: of removing it. I won't commit (because I want you to consider what I have written in case it would change your mind) but if you are all in agreement then feel free to commit.