api: temporarily remove the warning about sync-context in requestConnection(). (#5738) (#5752)

Contrary to #5736, we will still keep the sync-context requirement of
requestConnection(), because it prevents API fragmentation.
PickFirstLoadBalancer is the only known violator.  We will fix it on
master, but we don't want to make that change on 1.21.x because the
release is soon.  We simply remove the warning in this release so that
users won't be annoyed.

This supersedes #5736

Author: zhangkun83

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

@@ -927,7 +927,6 @@ public NameResolverRegistry getNameResolverRegistry() {
    *
    * @since 1.2.0
    */
-  @ThreadSafe
   @ExperimentalApi("https://github.com/grpc/grpc-java/issues/1771")
   public abstract static class Subchannel {
     /**
@@ -945,8 +944,7 @@ public abstract static class Subchannel {
     /**
      * Asks the Subchannel to create a connection (aka transport), if there isn't an active one.
      *
-     * <p>It should be called from the Synchronization Context.  Currently will log a warning if
-     * violated.  It will become an exception eventually.  See <a
+     * <p>It should be called from the Synchronization Context.  See <a
      * href="https://github.com/grpc/grpc-java/issues/5015">#5015</a> for the background.
      *
      * @since 1.2.0

core/src/main/java/io/grpc/internal/ManagedChannelImpl.java

@@ -1522,7 +1522,6 @@ public void run() {
 
     @Override
     public void requestConnection() {
-      logWarningIfNotInSyncContext("Subchannel.requestConnection()");
       subchannel.obtainActiveTransport();
     }