Remove /activity/ page

[mrduncan]

The https://sentry.sentry.io/activity/ page hasn't been linked to from anywhere for a long time and has a lot of missing data even when it's loaded. We should clean things up by removing it.

Tasks

Beta Give feedback

1. Send comms #67887
   Type: Support Request +1
   
2. Remove code after 2024-05-01 #67888
   Component: API +1
   

[vartec]

This page/endpoint are still being used, 423 calls from 16 organizations in last two weeks, including one organization very actively using it (325 calls in last two weeks).

I'll follow up with API team for a plan to properly deprecate.