remove navbar.md and its supporting logo

It was added in late 2021 following https://cuelang.org/issue/1101,
allowing Gitiles, GerritHub's web UI for git, to show a CUE logo
on pages such as:

    https://review.gerrithub.io/plugins/gitiles/cue-lang/cue/+/refs/heads/master/README.md

It's worth noting that the logo is rather low quality, and it doesn't
appear in a particularly consistent way with the rest of the web UI.
The CUE logo also never worked for common pages such as:

    https://review.gerrithub.io/plugins/gitiles/cue-lang/cue/
    https://review.gerrithub.io/plugins/gitiles/cue-lang/cue/+/refs/heads/master
    https://review.gerrithub.io/plugins/gitiles/cue-lang/cue/+/refs/tags/v0.9.1

Moreover, ever since we have been on GerritHub, the vast majority of
CUE users and contributors use GitHub to navigate the git source,
and not GerritHub via Gitiles.

The extra file in the root of the git repository seems not worth it.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I1269967a5057d4ac196c65ccdade70216ba29b94
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1196367
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>