Delete picture from the photo library using the command line?
You can either search using the Finder: 

kind:heic


Or, from the command-line - mdfind uses Spotlight:

mdfind -name 'heic'


Or, normal unix find:

 find /path/to/search/in -iname "*.heic"


You can then use that output to do as you please. Rather than deleting, I agree with Seamus that you could move them first, and confirm no issues with the Photo's library.

eg. to move them:

find /path/to/search/in -iname "*.heic" -exec mv {} /path/to/move/to/ \;