Remove a image from a ImageCollection if a band doesn't exist

I’m using the gee python api. The Hansen & Al. glad alert dataset is currently broken and the 20 last images are missing the ‘conf19’ band. In my process I need to use it anyway as a mosaic so I need to create a new Image Collection that contains all the image that have the ‘conf19’ band.

all_alerts  = ee.ImageCollection('projects/glad/alert/UpdResult') 

#need to filter the images that miss the 'conf19' band here 

var alerts_2019 = all_alerts.select('conf19').mosaic().clip(country);

How can I drop all the images that doesn’t contains a specific band ?