Replace UnionOfIntervals by RealSet

[mkoeppe]

Follow-up from #16063, which added missing features of UnionOfIntervals to RealSet

CC: @JohnCremona @tscrim

Component: geometry

Issue created by migration from https://trac.sagemath.org/ticket/31930

[JohnCremona]

comment:1

Just to reiterate what I said at #16063: I would only support this if there was no slowdown in the use of UnionOfIntervals for its intended use in computing height bounds on elliptic curves. I also do not intend to spend any time myself on this (to me) unnecessary proposed work.