Remove bfdwin.c

Remove bfdwin.c

In commit b86d3af60ffc and 0ab0435fe672 I fixed SIGBUS errors found by
oss-fuzz now that --with-mmap defaults to enabled.  It turns out there
are further problems with the aout mmap code: aout_read_minisymbols
returns the external symbol array, which is later freed by nm.c.  If
the array is mmaped you can't free it.  Now this could be fixed by
making aout minisymbols an array of pointers, but I figure there's not
much point in expending effort on that.  So delete the aout mmap
support along with bfdwin.c and get_section_contents_in_window.