osd/ECBackend: clean up assert(r==0) in continue_recovery_op.

After the commit(d9106ce5e4437ab02), the assert(r==0) is no longer
necessary.
Merge pull request #2194 from majianpeng/fix1

osd/ECBackend: clean up assert(r==0) in continue_recovery_op.

Reviewed-by: Samuel Just <sam.just@inktank.com>