Fix OpenSslClientSessionCache remove (#14366)

Motivation:
Loop was adding `nativeSslSession` to the `toBeRemoved` list, instead of
the loop variable `sslSession`.

Modification:
Add the loop variable instead, because the `nativeSslSession` can very
likely be null at that point, and is specifically also not the session
we want to remove.

Result:
Fewer bugs in SSL session caching

handler/src/main/java/io/netty/handler/ssl/OpenSslClientSessionCache.java

@@ -99,7 +99,7 @@ boolean setSession(long ssl, OpenSslSession session, String host, int port) {
                     if (toBeRemoved == null) {
                         toBeRemoved = new ArrayList<NativeSslSession>(2);
                     }
-                    toBeRemoved.add(nativeSslSession);
+                    toBeRemoved.add(sslSession);
                 }
             }