extra: remove unnecessary second level functions

do_strptime() and do_strftime()

I don't see any reason why src/libextra/time.rs has two functions that just call a very similar function, which only use is to be called by them.
strftime() just calls do_strftime()
strptime() just calls do_strptime()
and the do_functions aren't called by anyone else.
the parameters and return types are exactly the same in both levels, so I just have removed the second layer.

Unless there is a need for that second level.