Remove unnecessary color attributes from navigation block

[jorgefilipecosta]

To have a text color setting in the navigation block, we relied on four color attributes. In other blocks, we only use two attributes. I think we had some redundancy in the attributes.
We also used some inline styles even when a user selects a preset color; this may colors not work as expected when switching the color associated with a given class.
This PR also tries to simplify the code by relying on when the new useColors hook. I noticed that I needed to fix a small thing in useColors and that change is here for testing purposes, but we should merge it in a different PR.

It also solves some bugs:

• When a color we add navigation block and don't choose any text color, we save null in one of the attributes: <!-- wp:navigation-menu {"textColorCSSClass":null} -->. This pollutes the markup and is unnecessary.
• When we create a navigation block, select a preset color, save the post and later change the value of color associated with a given class (e.g: using the customizer in 2020 theme to change the primary color ), after the reload the color indication shows the previous color while the block now has a new color. See the image bellow.
• Following the previous scenario, when changing a color associated with a given class, in the frontend, the new color is applied as expected. However, the markup of the nav element still applies an inline style with the previous color value. This inline style seems unnecessary.

Description

How has this been tested?

I created multiple navigation blocks.
In some, I chose a preset color; in other, I picked a named color (at least one had accent color on 2020 theme).
I verified both blocks look as expected on the frontend and backend.
I changed the accent color on the customizer and verified the blocks correctly reflected the new color, including on the color indicator.

[jorgefilipecosta]

cc: @draganescu, @retrofox, @getdave

[draganescu]

Everything works as described.

Tested locally with by switching colors (primary, accent and custom) alternating usage of the control in the navigation block and the options in customizer.

Codewise we're having a cleaner approach with useColors.