Delete `ln -s /bin/true /sbin/initctl` line.

Official Ubuntu image replaces /sbin/initctl with /bin/true,
so it's no longer neccesary for us
to execute `ln -s /bin/true /sbin/initctl` in the `Dockerfile`.
Merge pull request #1 from kazu634/id/1

/sbin/initctlを/bin/trueに置換するのをやめる