[feat] Remove `Symbol.for('request-received.startAt')`

Not sure why this was added. If we want to store a start time in the `context, we should add it to the public API.

Start by locating Symbol.for('request-received.startAt') in the X-Response-Time middleware and read how the context is used around it. Check whether existing tests cover this symbol or start-time behavior. Done means the unexplained symbol is removed, or the intended public context API is clearly defined before implementation proceeds.

Written by the indexing model from the issue text.