Drop unused skip_abs option for Expr.compute_leading_term and calculate_series

    This was introduced in 89c7626 and then this option was unused since 9c46974
Drop obsoleted docs about skip_log option of compute_leading_term()

    This was replaced by logx option in 9c46974d
Merge pull request #2438 from skirpichev/misc-calculate_series

Cleanup some options in Expr.compute_leading_term and calculate_series