I want to remove the custom patch applied to the cloud.
What do you mean with remove ? If you want to revert it , use this :
git apply -R <path to name-of-the-patch.patch>
If you applied a patch ,definetly you have added it with composer , so when the patch was generated , he modified the vendor .
If , for example you have made a patch that have modified
vendor/magento/module-catalog , you have to remove
rm -rf vendor/magento/module-catalog
Open composer.json , find the correct name of the patch applied in composer.json and remove the line.
composer install to regenerate the composer.lock and vendor.