execinfrapb: remove IndexSkipTableReaderSpec

The `indexSkipTableReader` was deleted in cockroachdb#51178 but the distsql spec
has stuck around for a few versions. I think we can delete the spec now,
too. If someone wants to dust off cockroachdb#39668 they can bring it back.

Release note: None

pkg/sql/execinfrapb/processors_sql.proto

@@ -138,36 +138,6 @@ message FiltererSpec {
   optional Expression filter = 1 [(gogoproto.nullable) = false];
 }
 
-// IndexSkipTableReaderSpec is the specification for a table reader that
-// is performing a loose index scan over rows in the table. This means that
-// this reader will return distinct rows from the table while using the index
-// to skip unnecessary rows. This reader is used for different optimizations
-// when operating on a prefix of a compound key.
-message IndexSkipTableReaderSpec {
-  optional sqlbase.TableDescriptor table = 1 [(gogoproto.nullable) = false];
-  // If 0, we use the primary index. If non-zero, we use the index_idx-th index,
-  // i.e. table.indexes[index_idx-1]
-  optional uint32 index_idx = 2 [(gogoproto.nullable) = false];
-
-  reserved 3;
-  repeated roachpb.Span spans = 8 [(gogoproto.nullable) = false];
-
-  // This field used to be a visibility level of the columns that should be
-  // produced. We now always produce all columns (public and not public).
-  reserved 4;
-
-  optional bool reverse = 5 [(gogoproto.nullable) = false];
-
-  // Indicates the row-level locking strength to be used by the scan. If set to
-  // FOR_NONE, no row-level locking should be performed.
-  optional sqlbase.ScanLockingStrength locking_strength = 6 [(gogoproto.nullable) = false];
-
-  // Indicates the policy to be used by the scan for handling conflicting locks
-  // held by other active transactions when attempting to lock rows. Always set
-  // to BLOCK when locking_strength is FOR_NONE.
-  optional sqlbase.ScanLockingWaitPolicy locking_wait_policy = 7 [(gogoproto.nullable) = false];
-}
-
 // JoinReaderSpec is the specification for a "join reader". A join reader
 // performs KV operations to retrieve specific rows that correspond to the
 // values in the input stream (join by lookup). The output can optionally