Remove filterFirst/filterLast handling

Cove coverage complains it isn't tested, it is a bit involved to
actually test, and we actually don't need (at least not yet) this
method in those cases. In other words, this was kind of dead code,
so removing with assertions to prevent future misuse.

Author: pcmanus

src/java/org/apache/cassandra/io/sstable/format/trieindex/TrieIndexSSTableReader.java

@@ -271,24 +271,20 @@ protected RowIndexEntry getPosition(PartitionPosition key, Operator op, boolean
     private RowIndexEntry getApproximatePosition(PartitionPosition key, Operator op, boolean isLeftBound)
     {
         assert op == GT || op == GE;
-
-        PartitionPosition searchKey;
-        Operator searchOp;
-
-        if (filterLast() && last.compareTo(key) < 0)
-            return null;
-        boolean filteredLeft = (filterFirst() && first.compareTo(key) > 0);
-        searchKey = filteredLeft ? first : key;
-        searchOp = filteredLeft ? GE : op;
+        // We currently only need this method in contexts where neither early opening nor zero copy transfer are used,
+        // which means we don't have to worry about `filterFirst`/`filterLast`. We could expand that method to support
+        // those, but it's unclear it will ever be needed, and this would require proper testing, so leaving aside for now.
+        assert openReason != OpenReason.MOVED_START : "Early opening is not supported with this method";
+        assert !sstableMetadata.zeroCopyMetadata.exists() : "SSTables with zero copy metadata are not supported";
 
         try (PartitionIndex.Reader reader = partitionIndex.openReader())
         {
-            return reader.ceiling(searchKey, (pos, assumeNoMatch, compareKey) -> {
+            return reader.ceiling(key, (pos, assumeNoMatch, compareKey) -> {
                 // The goal of the overall method, compared to `getPosition`, is to avoid reading the data file. If
                 // whatever partition we look at has a row index (`pos >= 0`), then `retrieveEntryIfAcceptable` may
                 // read the row index file, but it will never read the data file, so we can use it like in `getPosition`.
                 if (pos >= 0)
-                    return retrieveEntryIfAcceptable(searchOp, compareKey, pos, assumeNoMatch);
+                    return retrieveEntryIfAcceptable(op, compareKey, pos, assumeNoMatch);
 
                 // If `assumeNoMatch == false`, then it means we've matched a prefix of the searched key. This means
                 // `pos` points to a key `K` in the sstable that is "the closest on" to `searchKey`, but it may be