Duplicate partial inclusion in head.html and head-end.html

@aidanranney: this is a [Docsy migration followup](https://github.com/grpc/grpc.io/issues/613) question concerning these issues:

- HTML head meta elements are duplicated #745
- Homepage has empty title/name for opengraph, twitter, and itemprop #744

Investigating this a bit I noticed the following:

(1) https://github.com/grpc/grpc.io/blob/main/layouts/partials/hooks/head-end.html contains:
```html
{{ partial "google-analytics.html" . }}
{{ partial "meta.html" . }}
{{ partial "favicons.html" . }}
```

(2) But https://github.com/grpc/grpc.io-docsy/blob/e95da39c9933afe159b74b0d6b5b71407aab7d7d/layouts/partials/head.html:
- Also contains
- `` tags
- A partial for favicons
- A partial for Google analytics
- As well as an include of `head-end.html` at the end of the file.

I'm sure that the duplication wasn't intentional, but what was the original intent? Was `head-end.html` created by mistake, or was the intent to customize `head.html` (by removing the elements that were being added to `head-end.html`)?

/cc @nate-double-u @celestehorgan