bfd/COFF: drop link_add_one_symbol() hook

The need for this has disappeared with dc12032bca08 ("Remove m68k-aout
and m68k-coff support"); avoid the unnecessary indirection.

Sadly, with ld/pe-dll.c using the wrapper, the removal requires moving
the declaration out of libcoff.h, to properly export the underlying BFD
function.