chore(core): remove net/trace event log

[xqqp]

In c4bb411 there was an effort to replace net/trace with spans from OpenCensus. A few remnants of net/trace still remain, this PR removes them.

The trace package exposes two endpoints: /debug/requests and /debug/events. They contain some incomplete data regarding raft indexes and schema loading. The documentation also does not refer to the endpoints or the event log at all.

The documentation however incorrectly refers to the flag expose_trace (https://docs.dgraph.io/learn/howto/retrieving-debug-information#metrics-information), that is meant for the event log, but describes the flag as controlling the /debug/varsendpoint.

This PR does the following:

• replace golang.org/x/net/context with stdlib context package
• remove all uses of golang.org/x/net/trace
• remove expose_trace flag from code and docker compose files
• demote golang.org/x/net to an indirect dependency

Regardless of this PR, the documentation should be updated to not associate the expose_trace config option with the /debug/vars endpoint

[xqqp]

The Trunk Code Quality check complains about formatting in the compose files. I don't have trunk installed. Can a maintainer take a look?