Remove target_close

Remove target_close

I noticed that target_close is only called in two places:
solib-svr4.c, and target_ops_ref_policy::decref.

This patch fixes the former by changing target_bfd_reopen to return a
target_ops_up and then fixing the sole caller.  Then it removes
target_close by inlining its body into the decref method.

The advantage of this approach is that targets are now automatically
managed.

Regression tested on x86-64 Fedora 38.

Approved-By: Andrew Burgess <aburgess@redhat.com>