bfd: simplify _bfd_{link,write,discard}_section_stabs() interface

... as well as that of _bfd_stab_section_offset(): As sec_info is now
hanging off of sec, there's no need for the extra 4th parameter anymore.
Along these line struct struct coff_section_tdata's stab_info member then
isn't needed anymore either.

Furthermore there also hasn't been a good reason to have the caller of
_bfd_link_section_stabs() set sec_info_type.