system_block_view_system_main_block_alter

system_block_view_system_main_block_alter(array &$build, BlockPluginInterface $block)

function system_block_view_system_main_block_alter(array &$build, BlockPluginInterface $block) {
  // Contextual links on the system_main block would basically duplicate the
  // tabs/local tasks, so reduce the clutter.
  unset($build['#contextual_links']);
}