deps: remove replace directive for etcd from go.mod

[loderunner]

Checklist

=========

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Internal Checklist

• I am requesting a review from my own team as well as the owning team
• I have a plan in place for the monitoring of the changes that I am making (this can include new monitors, logs to be aware of, etc...)

Changes

• Remove replace directive from go.mod for github.com/coreos/etcd.

Rationale

The vulnerability this was trying to avoid has been patched in 3.4.10, 3.3.23. And anyway, it seems we don't rely on github.com/coreos/etcd anymore.

Screenshots

❯ go mod why  github.com/coreos/etcd
# github.com/coreos/etcd
(main module does not need package github.com/coreos/etcd)