book-snippets.py: stop creating "image of music" txt files

Stop creating .txt files with the content "image of music". We now generate .texi with

@image{lilypond/67/lily-393eafe4,,,[image of music],}

so the "image of music" string is present in the textual info output.

Removes code authored in commit 215ba296.

N.B. Earlier versions of this script will reject a database created by this version, because they will be looking for .txt files where none exist.