It can be dropped those caches when the caches are no longer use by the following method. It will drop the unused caches on every 15 minutes :
echo "sync && echo 3 | tee /proc/sys/vm/drop_caches" > /usr/bin/kali_drop_caches
chmod +x /usr/bin/kali_drop_caches
(crontab -l ; echo "*/15 * * * * /usr/bin/kali_drop_caches") | crontab -
That's all! See you.