REF: Remove duplicate PET simulation data generation script

Remove duplicate PET simulation data generation script.

The script to simulate motion on a PET dataset was introduced in commit
f466d28 as part of the efforts to make PR
#88
easier to merge.

The removed script was inadvertently re-introduced in commit f0d38de as
the works to solve the conflicts in
#88 were resumed.

Author: jhlegarreta