Remove RegisterLicense from logical.Backend

It's almost certainly the wrong signature and nothing uses it currently
anyways.

logical/plugin/backend_client.go

@@ -79,16 +79,6 @@ type TypeReply struct {
 	Type logical.BackendType
 }
 
-// RegisterLicenseArgs is the args for the RegisterLicense method.
-type RegisterLicenseArgs struct {
-	License interface{}
-}
-
-// RegisterLicenseReply is the reply for the RegisterLicense method.
-type RegisterLicenseReply struct {
-	Error error
-}
-
 func (b *backendPluginClient) HandleRequest(ctx context.Context, req *logical.Request) (*logical.Response, error) {
 	if b.metadataMode {
 		return nil, ErrClientInMetadataMode
@@ -265,23 +255,3 @@ func (b *backendPluginClient) Type() logical.BackendType {
 
 	return logical.BackendType(reply.Type)
 }
-
-func (b *backendPluginClient) RegisterLicense(license interface{}) error {
-	if b.metadataMode {
-		return ErrClientInMetadataMode
-	}
-
-	var reply RegisterLicenseReply
-	args := RegisterLicenseArgs{
-		License: license,
-	}
-	err := b.client.Call("Plugin.RegisterLicense", args, &reply)
-	if err != nil {
-		return err
-	}
-	if reply.Error != nil {
-		return reply.Error
-	}
-
-	return nil
-}