[DOCS] Remove shadow replica reference (#50029)

Removes a reference to shadow replicas from the cat shards API docs
and a comment in cluster/routing/UnassignedInfo.java.

Shadow replicas were removed with #23906.

Author: jrodewig

docs/reference/cat/shards.asciidoc

@@ -138,5 +138,5 @@ These are the possible reasons for a shard to be in a unassigned state:
 `ALLOCATION_FAILED`::       Unassigned as a result of a failed allocation of the shard.
 `NODE_LEFT`::               Unassigned as a result of the node hosting it leaving the cluster.
 `REROUTE_CANCELLED`::       Unassigned as a result of explicit cancel reroute command.
-`REINITIALIZED`::           When a shard moves from started back to initializing, for example, with shadow replicas.
+`REINITIALIZED`::           When a shard moves from started back to initializing.
 `REALLOCATED_REPLICA`::     A better replica location is identified and causes the existing replica allocation to be cancelled.

server/src/main/java/org/elasticsearch/cluster/routing/UnassignedInfo.java

@@ -113,7 +113,7 @@ public enum Reason {
          */
         REROUTE_CANCELLED,
         /**
-         * When a shard moves from started back to initializing, for example, during shadow replica
+         * When a shard moves from started back to initializing.
          */
         REINITIALIZED,
         /**