Remove unused _get_file_size()

Since I404e43e2e6ffecb5131cdcc43df03b8da0e7f0f8 , _get_file_size() was
never used. So this patch removes it for the code cleanup.

Change-Id: I4d36c140a137f2a7a72315c6ab952d5ca4e414cc
1 file changed