Remove new_bfd_ref

Remove new_bfd_ref

For gdb_bfd_ref_ptr, gdb already had a convenience function like the
new gdb_ref_ptr::new_reference -- called new_bfd_ref.  This patch
removes it in favor of the new common function.

While doing this I also noticed that the comment for gdb_bfd_open was
incorrect (in a way related to reference counting), so this patch
updates the comment as well.

ChangeLog
2018-04-30  Tom Tromey  <tom@tromey.com>

	* symfile-mem.c (symbol_file_add_from_memory): Use new_reference.
	* gdb_bfd.h (new_bfd_ref): Remove.
	(gdb_bfd_open): Update comment.
	* gdb_bfd.c (gdb_bfd_open, gdb_bfd_fopen, gdb_bfd_openr)
	(gdb_bfd_openw, gdb_bfd_openr_iovec, gdb_bfd_record_inclusion)
	(gdb_bfd_fdopenr): Use new_reference.
	* exec.c (exec_file_attach): Use new_reference.