migration: Remove unused qemu_file_credit_transfer()

After this change, nothing abuses QEMUFile to account for data
transferrefd during migration.

Reviewed-by: Leonardo Bras <leobras@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20230515195709.63843-15-quintela@redhat.com>