remove unnecessary asserts

[gquintard]

We only use ctx->now.

Removing the asserts will allow users to call xkey.purge() from a backend context, if that's a too big of a footgun, we can use $Restrict

[dridi]

Can you add a test case? At least to make sure this is safe.

[gquintard]

of course

[gquintard]

new test pushed