Remove @state.parent assignment on commit

This piece of code was introduced on
67d8bb963d5d51fc644d6b1ca20164efb4cee6d7 , which was calling
`committed?` in the `transaction_state` before calling the `committed!`
method. However on 7386ffc781fca07a0c656db49fdb54678caef809, the
`committed?` check was removed and replaced by a `finalized?`, which
only checks if the state is not nil. Thus we can remove that line.

review @chancancode @tenderlove @wangjohn @rafaelfranca
@rafaelfranca I am removing it, in my next PR... Just wanna make sure I can remove that line safely in a isolated commit (to make blame, bisect easy)