Remove cover image ‘strong’ style

[johngodley]

The bold style on a cover image has no effect on the preview. Here the first word is made bold and has the same weight (300) as the second word:

The reason for this is that the cover block overrides the standard strong weight and specifically sets it to 300. There may have been a reason for this in the past but it's one of the few places in Gutenberg where strong is changed, and removing it fixes the problem. As the CSS is only scoped to the cover block within the rich text editor I don't think it will affect anything else.

Fixes #8202

How has this been tested?

1. Add a cover image and try to make the cover text bold
2. Note that bold text should look visually different, and using a browser inspector it has a weight of 600

Types of changes

Removes CSS override for .wp-block-cover .editor-rich-text strong

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[jasmussen]

Nice work. Ship it!