Doc: Fix description and documentation for link color support

[t-hamano]

What?

This PR updates the incorrect description and documentation of the block support link color. Specifically, remove explanations mainly related to the linkColor property.

Why?

My understanding is that for text color and background color, either of the following is stored as attributes:

• Text Color: textColor, style.color.text
• Background Color: backgroundColor, style.color.background

However, as far as the link color, it's only stored in style.elements.link.color.text, regardless of whether it is the custom color or not, and linkColor doesn't appear to be in use right now.

How?

In the handbook and in the schema of block.json, I have removed the description of linkColor. I have also updated the sample code presented in the Support section of the handbook to be correct.

Testing Instructions

This PR has no impact on the code.

[jorgefilipecosta]

The updates are correct. There is no linkColor color attribute added.
We can check that in:
packages/block-editor/src/hooks/style.js and packages/block-editor/src/hooks/color.js.