sstable: read_toc: get rid of file_path
sstable: read_toc: get rid of file_path

In preparation for closing the file in all paths,
get rid of the file_path sstring and just recompute
it as needed on error paths using the this->filename method.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla