[capink]

If you are familiar with the Action Chains plugin, you can achieve what you want:

• Copy past this custom action into the plugin's module editor
  

  from functools import partial
  
  from PyQt5.Qt import (QApplication, Qt)
  
  from calibre.gui2 import error_dialog
  
  from calibre_plugins.action_chains.common_utils import DoubleProgressDialog
  from calibre_plugins.action_chains.actions.base import ChainAction
  
  class Test(ChainAction):
  
      name = 'Trim Covers'
      support_scopes = True
  
      def trim_covers(self, db, book_ids, pbar):
  
          from calibre.utils.img import (
              image_from_data, image_to_data, remove_borders_from_image
          )
          
          pbar.update_overall(len(book_ids))
          
          for book_id in book_ids:
              cdata = db.new_api.cover(book_id)
              if cdata:
                  img = image_from_data(cdata)
                  nimg = remove_borders_from_image(img)
                  if nimg is not img:
                      cdata = image_to_data(nimg)
                      db.new_api.set_cover({book_id:cdata})
              msg = _('Trimming cover for book_id: {}'.format(book_id))
              pbar.update_progress(1, msg)
  
      def run(self, gui, settings, chain):
          db = gui.current_db
          book_ids = chain.scope().get_book_ids()
  
          callback = partial(self.trim_covers, db, book_ids)
          pd = DoubleProgressDialog(1, callback, gui, window_title=_('Trimming ...'))
  
          gui.tags_view.blockSignals(True)
          QApplication.setOverrideCursor(Qt.ArrowCursor)
          try:
              pd.exec_()
  
              pd.thread = None
  
              if pd.error is not None:
                  return error_dialog(gui, _('Failed'),
                          pd.error[0], det_msg=pd.error[1],
                          show=True)
          finally:
              QApplication.restoreOverrideCursor()
              gui.tags_view.recount()

• Create a chain containing the action you just added: Trim Covers.
• Assign a keyboard shortcut your want to the chain (Add/Modify Chains > Keyboard shortcuts)