msmtp-scripts: Remove as abandoning upstream; msmtp-queue works

I am upstream for msmtp-scripts and have decided to abandon the project.  Therefore
remove msmtp-scripts from OpenWrt -- there is already msmtp-queue which is 'good enough'
for the use cases where msmtp-scripts had any relevance.

This backports to 19.07 so that it doesn't become something folks are depending on.
Due to changes in lock behaviour it never worked in 18.04.x, so ditching it now
keeps it from being picked up again by the userbase.

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>