osl_open_table(): Remove pointless directory check.

osl_open_table(): Remove pointless directory check.

The subsequent mmap(2) call will fail if the directory does not exist
(or if the path is not a directory), so this check is not needed.

This removes the only user of the E_OSL_NOTDIR error code. We must
keep it in the errlist file though, because error codes must be
considered part of the API.