controller: remove controller grpc service

Remove the controller grpc service along with associated code related to
sessions or remote controllers.

Data types that are still used with complicated dependency chains have
been kept in the same package for a future refactor.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

monitor/commands/rollback.go

@@ -7,7 +7,6 @@ import (
 
 	controllerapi "github.com/docker/buildx/controller/pb"
 	"github.com/docker/buildx/monitor/types"
-	"github.com/pkg/errors"
 )
 
 type RollbackCmd struct {
@@ -38,9 +37,6 @@ COMMAND and ARG... will be executed in the container.
 }
 
 func (cm *RollbackCmd) Exec(ctx context.Context, args []string) error {
-	if ref := cm.m.AttachedSessionID(); ref == "" {
-		return errors.Errorf("no attaching session")
-	}
 	cfg := cm.invokeConfig
 	if len(args) >= 2 {
 		cmds := args[1:]