Use inds1, inds2 smartly in vec_approx_equal, or remove

[brookslogan]

If it continues to look like a major time contributor in major operations.

For data frames, we don't have to slice as much of later columns if an early column has already determined the result for a given entry to be FALSE or NA. We don't even need to slice later columns at all if the earlier entries have already determined the results for all entries. If this provides a substantial speed-up, we should compare speed vs. vec_equal and determine whether to use it in the abs_tol == 0 data frame case rather than vec_equal.

This was the original idea behind these arguments, but was never actually implemented. These args coincidentally provided a speedup as the implementation used vec_slice instead of df[<row indices>,]. If we can't actually improve performance in a real way using these args, they can be removed to simplify the interface and code.