Revert "fix cleanup handling in macho_symfile_read"

This patch indirectly causes a SEGV by creating a dangling pointer.
Reverting this patch while working on a clearer memory management
method for this part of the code.

gdb/Changelog:

        Revert:
        * machoread.c (macho_symfile_read): Assign first cleanup to
        'back_to'.