Removed Unnecessary Assignment for 'error' Var (#920)

`error = abs(f(a))` was declared on line 24 and line 32. It is unnecessary to have in both places.
I removed the second instance since it wastes resources to keep redefining the variable inside the for loop.
This fixes an [issue found by lgtm](https://lgtm.com/projects/g/TheAlgorithms/Python/snapshot/66c4afbd0f28f9989f35ddbeb5c9263390c5d192/files/maths/newton_raphson.py?sort=name&dir=ASC&mode=heatmap)