Trac #34410: make new composite elliptic-curve isogenies the default

Since #32744, Sage has a specialized class `EllipticCurveHom_composite`
for composite elliptic-curve isogenies which is designed to behave the
same as other, non-factored isogenies.

In this patch, we change the default composition for `EllipticCurveHom`
instances from generic formal composite morphisms to the specialized
implementation.

URL: https://trac.sagemath.org/34410
Reported by: lorenz
Ticket author(s): Lorenz Panny
Reviewer(s): Travis Scrimshaw

Author: Release Manager