Remove dead files.dat references from build config (closes #22)

files.dat was the pre-database on-disk scan format; nothing has
produced it since the persistent SQLite database landed. Drop the
stale references from Makefile clean, .gitignore and .dockerignore.
make clean still removes the binary, and .gitignore still covers the
actual database artifacts (*.sqlite, *.sqlite-shm, *.sqlite-wal). The
sole remaining files.dat mention is the historical entry in TODO.md.

Model: opus-4-8

TODO.md

@@ -31,7 +31,8 @@
 
 - remove the dead `files.dat` references from `Makefile`, `.gitignore`
   and `.dockerignore` (2026-09-21, branch `next`, closes
-  https://git.eeqj.de/sneak/sfdupes/issues/22)
+  https://git.eeqj.de/sneak/sfdupes/issues/22): `make clean` still
+  removes the binary; the `.sqlite`/`-shm`/`-wal` ignores remain.
 
 - fix the lint-image pin comments and `FROM` form in `Dockerfile` and
   `Dockerfile.lint` (2026-08-10, branch `next`, closes