Remove bracket-int notation in Subsetting&Filtering

[eswan18]

In the subsetting and filtering notebook, we show a way of selecting rows by using bracket notation on the dataframe directly:

However, it is not only misleading (as pointed out above), but it is actually ambiguous – in certain cases (many, I think), it will select columns and not rows.

Note that in this case, passing an integer selected a column (not a row). For clarity and simplicity, I believe we should not even introduce this syntax as an option.

[markroepke]

Good catch. I'll remove it.

[eswan18]

Looks good