bfd: replace _bfd_merge_sections() hook with simple boolean

There's no need for a hook; what needs doing is uniform, the question is
only whether to perform any merging (i.e. whether other parts of a backend
are capable of dealing with the effects).

Where _bfd_nolink_bfd_merge_sections() was used, false is hardcoded. For
ELF no real target override is permitted; true is hardcoded except for the
cases where bfd_generic_merge_sections() was used as the hook function
before.