gh-35494: avoid computation of Conway polynomials when comparing elliptic-curve morphisms
...to make things run faster.
Example:
```sage
sage: p = 2**127 - 1
sage: F.<i> = GF((p,2), modulus=[1,0,1])
sage: E = EllipticCurve(F, [1,0])
sage: %prun -s cumtime E.frobenius_isogeny()^2 ==
E.scalar_multiplication(-p)
```
Sage 10.0.beta8: `CPU times: user 6.02 s, sys: 3.28 ms, total: 6.02 s`
This branch: `CPU times: user 3.89 s, sys: 3.08 ms, total: 3.89 s`
URL: https://github.com/sagemath/sage/pull/35494
Reported by: Lorenz Panny
Reviewer(s): Kwankyu Lee, Lorenz Panny
