sheep/plain_store: remove checking length in default_read_from_path

Checking length after reading object fileis not a good idea.
Disk auto unplugging will be occurred under one of the two
situations:

1. A new version dog with ledger object supported sends request to
   an old version sheep withou ledger object supported

2. Object file is modified by the third party and the length of it
   is not the original size

So, I suggest remove checking length.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>