Change target_bfd_reopen to take a gdb_bfd_ref_ptr

While looking at Andrew's recent target sections series, I saw that
target_bfd_reopen took a "bfd *", leading to a call to new_reference.
However, because the only caller of target_bfd_reopen is already using
gdb_bfd_ref_ptr, this code can be simplified and the explicit call to
new_reference can be removed.

gdb/ChangeLog
2021-02-22  Tom Tromey  <tromey@adacore.com>

	* solib-svr4.c (enable_break): Update.
	* bfd-target.c (class target_bfd) <target_bfd>: Change parameter
	type.
	(target_bfd_reopen): Change parameter type.
	* bfd-target.h (target_bfd_reopen): Change parameter type.