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
Is `parent` still being set outside the class? I believe it is not. So we can change from `attr_accessor` to  `attr_reader` or if we don't read it outside the class we should just remove the methods and keep this inside the instance.