doc(bigquery): add code sample for scripting

[tswast]

This code sample initiates a scripting job and then demonstrates how to
fetch the results and the child jobs.

Removes system test for scripting, as it's redundant with the code
sample.

[plamut]

Looks good, apart from the remark @tseaver gave.

(disclaimer: I wrote the original system test that this sample is based on)

[plamut]

Looks good. 👍