How to remove 'destination' query paramater from link URLs in Drupal 8
Before proceeding, realize that although this might be better for SEO; it might be negative for user experience. Personally I'd hate it if I were to comment on a node and after the login I have to navigate back to that node. You might want to perform some session magic to redirect the user back to where he came from. 

The answer to your question involves a custom module, with a Path Processor service. In addition to the default module files (mymodule.module and mymodule.info.yml), you'll need mymodule.services.yml and src/PathProcessor/MyModulePathProcessor.php.

The services file will expose your path processor to Drupal with this code:

services:
  mymodule.path_processor:
    class: Drupal\mymodule\PathProcessor\MyModulePathProcessor
    # Optionally provide arguments.
    #arguments: ['@config.factory', '@entity_type.manager', '@current_user']
    tags:
      - { name: path_processor_outbound, priority: -10} # Low priority acts last.


This 'subscribes' the class to the path_processor_outbound event. 

The Outbound path processor should look something like this:

&lt;?php

namespace Drupal\mymodule\PathProcessor;

use Drupal\Core\PathProcessor\OutboundPathProcessorInterface;
use Drupal\Core\Render\BubbleableMetadata;
use Symfony\Component\HttpFoundation\Request;

/**
 * Class MyModulePathProcessor.
 */
class MyModulePathProcessor implements OutboundPathProcessorInterface {

  /**
   * {@inheritdoc}
   */
  public function processOutbound($path, &amp;$options = [], Request $request = NULL, BubbleableMetadata $bubbleable_metadata = NULL) {
    if ($path == 'user/login') {
      unset( $options['query']['destination'] );
    }
    return $path;
  }

}