Remove custom admin header spacing modifications.

[StevenDufresne]

As suggested by @ryelle in WordPress/wordcamp.org#1410 (comment), this PR removes custom CSS styles that were added when the WordPress.org header was sticky.

This fixes issues on WordCamp sites where the Admin bar is positioned on top of the site's header.

Example

Before	After

Related: As suggested by @ryelle in WordPress/wordcamp.org#1410.

How to Test

Edit: Apply WordPress/wporg-parent-2021#162 first.

1. Apply these changes in your sandbox.
2. Navigate around WordPress.org and WordCamp sites and make sure the spacing between the header and the admin bar is expected. Not overlapping, or has an extra space.

[ryelle]

It looks like something is still removing the callback on the child-theme sites:

[StevenDufresne]

Ah, my apologies. I tested this incorrectly. The parent theme is removing the styles.

Opened up a PR WordPress/wporg-parent-2021#162.

[ryelle]

Yeah, testing it is a pain since you need to sync to both wporg and wordcamp. This looks good now in combination with the parent theme change 👍🏻

You mentioned in the issue

It seems like some WordCamp sites have already shipped fixes so whatever we do here, we'll need to reach out to those organizers ahead of time.

I did notice in spot-checking the upcoming camps in November that Nairobi & Larissa will have extra whitespace when this merges. The rest, at least in that group, seemed fine.