Remove expiration/TTL commands that are not supported by Valkey

Support for this commands may come in the future[1].
But it will take some time, so for now it's better
to drop them.

This is a breaking change for 6.1.

Close #78

[1]: valkey-io/valkey#640

Signed-off-by: Salvatore Mesoraca <[email protected]>

Author: aiven-sal