Remove abuse of predefined x in doctests

<div id="comment:0"></div>

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).



CC:  @tscrim @orlitzky

Component: **refactoring**

_Issue created by migration from https://trac.sagemath.org/ticket/32721_

