this deletes one post with iMacro (on the current Facebook site). You can delete all of them using iMacro's loop button.
Before launching the script, visit: https://www.facebook.com/<YOUR-USERNAME>/allactivity?privacy_source=activity_log&log_filter=cluster_11
and scroll down to load some posts.
VERSION BUILD=8810214 RECORDER=FX
SET !ERRORIGNORE YES
' Select "delete" for the first post
TAG POS={{!loop}} TYPE=A ATTR=aria-label:"Allowed on Timeline"
WAIT SECONDS=1
TAG POS={{!loop}} TYPE=SPAN ATTR=TXT:Delete
' Confirm the deletion
WAIT SECONDS=1
TAG POS=1 TYPE=BUTTON FORM=class:_s ATTR=TXT:Delete<SP>Post
WAIT SECONDS=3