CiviCRM core now includes Flexmailer: how to remove old flexmailer extension?
Here's how I did it: Delete the cache files
From within the site's extensions dir:
EXT_DIR=/var/www/mysite.org/sites/default/files/civicrm/ext/
CONFIG_AND_LOG_DIR=/var/www/mysite.org/sites/default/files/civicrm/ConfigAndLog
BACKUP_DIR=~/just-in-case

mkdir -p &quot;$BACKUP_DIR&quot;
mv org.civicrm.flexmailer $BACKUP_DIR/.
cd $CONFIG_AND_LOG_DIR
rm Cache* # Remove cached container/ExtLoader files.
cv flush # runs cleanly
cv ext:upgrade-db