fix clear_presets() match pattern (RHBZ #1349664)
- is a special character in lua matches; this effectively turns
the match into `^8.*%.preset$`, meaning this function will wipe
not just 80-*.preset but 8*.preset, including wiping out some
other important preset files (like 85-display-manager.preset),
unfortunately. See bug report for more details.
Signed-off-by: Dennis Gilmore <dennis@ausil.us>