Remove deprecated `context` argument

[haacked]

As stated in #54, the context argument is ignored and never used. However, to remove it immediately would be a breaking change. So first, we deprecated it in #182.

So this issue is to here to remind us to follow-up and remove the argument after a sufficient time-period. I don't know if PostHog has a standard deprecation approach in place that states how long to wait. I'll ask around. Otherwise, I'll give it a few months.