db: drop hints that produce too many virtual sstables

Previously we kept around hints that were creating too many
virtual sstables instead of dropping them. This change drops them.

Fixes #4055.

compaction.go

@@ -2113,9 +2113,9 @@ func checkDeleteCompactionHints(
 		}
 		if filesDeletedByCurrentHint < 0 {
 			// This hint does not delete a sufficient number of files to warrant
-			// a delete-only compaction at this stage. Add it to unresolvedHints
-			// so it can be re-evaluated later.
-			unresolvedHints = append(unresolvedHints, h)
+			// a delete-only compaction at this stage. Drop it (ie. don't add it
+			// to either resolved or unresolved hints) so it doesn't stick around
+			// forever.
 			continue
 		}
 		// This hint will be resolved and dropped.

testdata/compaction_delete_only_hints

@@ -677,7 +677,7 @@ o: (o, .)
 maybe-compact
 ----
 Deletion hints:
-  L0.000005 l-m seqnums(tombstone=11-11, file-smallest=10, type=point-and-range-key)
+  (none)
 Compactions:
   (none)
 
@@ -701,4 +701,4 @@ o: (o, .)
 
 get-hints
 ----
-L0.000005 l-m seqnums(tombstone=11-11, file-smallest=10, type=point-and-range-key)
+(none)