Deprecation of compute_leading_term

[0sidharth]

The compute_leading_term method is being deprecated in #21837. as_leading_term can be used instead.

The individual _eval_as_leading_term methods are quite cheap, some of them involve a call to limit, but that is generally on trivial args and would not take much time. compute_leading_term calculates series first (which itself would be relatively expensive if the arg of the function is not trivial), and then calls as_leading_term.
The possible simplification of the argument of the function to its series expansion (which itself could be slow in the same cases _eval_as_leading_term is slow due to the limit call), does not give any visible speed improvements over directly calling as_leading_term, so this method is no longer needed. It was used at 2 places in the codebase, which have been changed.

This is also the deprecation removal tracking issue.