remove unnecessary and undocumented source.sh and target.sh

[deitch]

These were used in the old bash-based version. In the newer version (from 1.0.0-rc* onwards):

• they were not documented anywhere
• they depended on a hard-coded path to /scripts.d/, which only made sense in a container context
• source.sh is unneeded as you can use post-backup scripts, which include an example of adding data to the backup file
• target.sh is unneeded, as you have the option --filename-pattern