Rachana: more 'akhn' fixes

Signed-off-by: Rajeesh K Nambiar <rajeeshknambiar@gmail.com>
Revert "Port Rachana to mlm2 opentype spec while keeping mlym compatible."

This reverts commit fe5b99faff65e1058e1f5164e55d9522b6e9293f.
We can hopefully come up with a better patch supporting both specs.
Rachana: fix 'psts' rule for യ്തു

Signed-off-by: Rajeesh K Nambiar <rajeeshknambiar@gmail.com>