Remove abuse of predefined x in doctests #32721
Description
For example in src/sage/rings/number_field/number_field.py
sage: K.<a> = NumberField(x^2 - 2)
This leads to numerous errors when sage.symbolic is not available (#32601).
Component: refactoring
Issue created by migration from https://trac.sagemath.org/ticket/32721