api: Remove obsolete TODO in EquivalentAddressGroup#toString (#12627)

### Background

EquivalentAddressGroup is intended to represent a small set of addresses
for the same endpoint (e.g., IPv4/IPv6). A long-standing TODO in
EquivalentAddressGroup#toString() suggested adding summarization for
very large address lists.

During review, maintainers noted that large address lists within a
single EquivalentAddressGroup are no longer expected in current designs,
and the historical behavior that could create a very large group (e.g.,
by aggregating many groups into one) has been removed. As a result, the
TODO is now obsolete.

### Changes

Remove the obsolete TODO comment in EquivalentAddressGroup#toString().

Add a brief explanatory comment capturing why summarization is no longer
needed.

### Purpose

Avoid leaving stale TODOs that imply work is still required.

Align the codebase with the intended semantics and current behavior of
EquivalentAddressGroup.

### Note

This change is comment-only and does not alter any runtime behavior or
output format.

Fixes #12593

---------

Author: stagegrowth

api/src/main/java/io/grpc/EquivalentAddressGroup.java

@@ -122,7 +122,9 @@ public Attributes getAttributes() {
 
   @Override
   public String toString() {
-    // TODO(zpencer): Summarize return value if addr is very large
+    // EquivalentAddressGroup is intended to contain a small number of addresses for the same
+    // endpoint(e.g., IPv4/IPv6). Aggregating many groups into a single EquivalentAddressGroup
+    // is no longer done, so this no longer needs summarization.
     return "[" + addrs + "/" + attrs + "]";
   }