Remove Bad Behavior cache plugin integration (#1071)

* Remove Bad Behavior cache plugin integration

The Bad Behavior plugin (bad-behavior.ioerror.us) is long gone, and the
integration was non-functional anyway: wp_supercache_badbehaviour_include()
had an inverted guard (require_once only ran when the file path was false),
so it never loaded Bad Behavior and would require_once '' on the false branch.

Removes plugins/badbehaviour.php entirely. The plugin file was auto-loaded
via glob() in wp-cache-phase1.php and self-registered all its hooks, so
deleting the file removes the feature completely — no loader list to update.

* Drop changelog entry (changelogger no longer used)

Author: donnchawp