Fix bugnote_clear_cache()

bugnote_clear_cache() only removed the note from $g_cache_bugnotes_by_id
if it has also been stored in $g_cache_bugnotes_by_bug_id, but failed in
the scenario where the bug-level cache has not been populated.

Adding a specific unset() call to ensure the bugnote is actually removed
from both caches.

Fixes 0027217