[WebUI]: Remove unneeded shared overrides

[fallaciousreasoning]

Previously, we needed to override upstream's default WebUI styles. However, since brave/brave-core#30567 has landed we override the default colors in C++ to allow for dynamic theming.

This means we should be able to get rid of our specific overrides and let colors flow through.

Test Plan

1. Fonts should look the same on chrome://settings/ chrome://history chrome://password-manager
2. Accent color should be blurple by default in the above pages
3. It should be possible to change the accent color via chrome://settings/appearance, and after a refresh it should take effect

Pay particular attention to:

1. Checkboxes
2. Buttons
3. Radiobuttons
4. Links

Note: If you see a one of the above with the right color, that's enough to check that this is working, no need to check every single one.